Update flake8 to 3.7.5 (and add py3/py2 compat) (#10592)

This PR updates [flake8](https://pypi.org/project/flake8) from **3.7.3** to **3.7.5**.



<details>
  <summary>Changelog</summary>
  
  
   ### 3.7.5
   ```
   -------------------

You can view the `3.7.5 milestone`_ on GitLab for more details.

Bugs Fixed
~~~~~~~~~~

- Fix reporting of pyflakes &quot;referenced before assignment&quot; error (See also
  `GitLab!301`_, `GitLab503`_)


.. all links
.. _3.7.5 milestone:
    https://gitlab.com/pycqa/flake8/milestones/28

.. issue links
.. _GitLab503:
    https://gitlab.com/pycqa/flake8/issues/503

.. merge request links
.. _GitLab!301:
    https://gitlab.com/pycqa/flake8/merge_requests/301
   ```
   
  
  
   ### 3.7.4
   ```
   -------------------

You can view the `3.7.4 milestone`_ on GitLab for more details.

Bugs Fixed
~~~~~~~~~~

- Fix performance regression with lots of ``per-file-ignores`` and errors
  (See also `GitLab!299`_, `GitLab501`_)


.. all links
.. _3.7.4 milestone:
    https://gitlab.com/pycqa/flake8/milestones/27

.. issue links
.. _GitLab501:
    https://gitlab.com/pycqa/flake8/issues/501

.. merge request links
.. _GitLab!299:
    https://gitlab.com/pycqa/flake8/merge_requests/299
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/flake8
  - Changelog: https://pyup.io/changelogs/flake8/
  - Repo: https://gitlab.com/pycqa/flake8
</details>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help docs test test_es test_no_es test_force_db tdd test_failed initialize_db populate_data update_deps update_db update_assets initialize update reindex flake8 shell debug
+.PHONY: help docs test test_es test_no_es test_force_db tdd test_failed initialize_db populate_data update_deps update_db update_assets initialize update reindex lint-codestyle shell debug
 
 IN_DOCKER = $(wildcard /addons-server-docker-container)
 
@@ -10,17 +10,14 @@ endif
 
 include $(SUB_MAKEFILE)
 
+
 help:
 	@echo "Please use 'make <target>' where <target> is one of the following commands."
 	@echo "Commands that are designed to be run in either the container or the host:"
 	@echo "  docs              to builds the documentation"
-	@echo "  flake8            to run the flake8 linter"
 	@$(MAKE) help_submake --no-print-directory
 
 	@echo "Check the Makefile to know exactly what each target is doing."
 
 docs:
 	$(MAKE) -C docs html
-
-flake8:
-	flake8 src/ services/ tests/

Makefile-docker

@@ -115,8 +115,7 @@ install_python_test_dependencies:
 	$(PIP_COMMAND) install --progress-bar=off --no-deps --exists-action=w -r requirements/prod_py${PYTHON_VERSION_MAJOR}.txt
 	$(PIP_COMMAND) install --progress-bar=off --no-deps --exists-action=w -r requirements/tests.txt
 
-install_python_dev_dependencies: install_python_test_dependencies
-	$(PIP_COMMAND) install --progress-bar=off --no-deps --exists-action=w -r requirements/flake8.txt
+install_python_dev_dependencies: install_python_test_dependencies setup-codestyle
 	$(PIP_COMMAND) install --progress-bar=off --no-deps --exists-action=w -r requirements/dev.txt
 	$(PIP_COMMAND) install --progress-bar=off --no-deps --exists-action=w -r requirements/dev_without_hash.txt
 	$(PIP_COMMAND) install --progress-bar=off --no-deps --exists-action=w -r requirements/docs.txt
@@ -177,4 +176,10 @@ perf-tests: setup-ui-tests
 	$(PIP_COMMAND) install --progress-bar=off --no-deps -r requirements/perftests.txt
 	locust --no-web -c 1 -f tests/performance/locustfile.py --host "http://olympia.test"
 
+setup-codestyle:
+	$(PIP_COMMAND) install --progress-bar=off --no-deps --exists-action=w -r requirements/codestyle_py${PYTHON_VERSION_MAJOR}.txt
+
+lint-codestyle:
+	flake8 src/ services/ tests/
+
 initialize: update_deps initialize_db update_assets populate_data

requirements/codestyle_common.txt

@@ -1,19 +1,9 @@
 -r system.txt
 
-# configparser is required by flake8
-configparser==3.7.1 \
-    --hash=sha256:c114ff90ee2e762db972fa205f02491b1f5cf3ff950decd8542c62970c9bedac \
-    --hash=sha256:df28e045fbff307a28795b18df6ac8662be3219435560ddb068c283afab1ea7a \
-    --hash=sha256:5bd5fa2a491dc3cfe920a3f2a107510d65eceae10e9c6e547b90261a4710df32
-# enum34 is required by cryptography, flake8, traitlets
-enum34==1.1.6 \
-    --hash=sha256:6bd0f6ad48ec2aa117d3d141940d484deccda84d4fcd884f5c3d93c23ecd8c79 \
-    --hash=sha256:8ad8c4783bf61ded74527bffb48ed9b54166685e4230386a9ed9b1279e2df5b1 \
-    --hash=sha256:2d81cbbe0e73112bdfe6ef8576f2238f2ba27dd0d55752a776c41d38b7da2850
 # flake8 is required by flake8-mutable
-flake8==3.7.3 \
-    --hash=sha256:806ec5785af23b4d9f3224a47cdd73714528923fccc95cc13cb1b74ba19c1820 \
-    --hash=sha256:b70f13ee944e536f22becfd716802b1f2fa4ca5ff7aea2c6423567569004f732
+flake8==3.7.5 \
+    --hash=sha256:c3ba1e130c813191db95c431a18cb4d20a468e98af7a77e2181b68574481ad36 \
+    --hash=sha256:fd9ddf503110bf3d8b1d270e8c673aab29ccb3dd6abf29bae1f54e5116ab4a91
 flake8-mutable==1.2.0 \
     --hash=sha256:ee9b77111b867d845177bbc289d87d541445ffcc6029a0c5c65865b42b18c6a6 \
     --hash=sha256:38fd9dadcbcda6550a916197bc40ed76908119dabb37fbcca30873666c31d2d5
@@ -33,7 +23,3 @@ pyflakes==2.1.0 \
 entrypoints==0.3 \
     --hash=sha256:589f874b313739ad35be6e0cd7efde2a4e9b6fea91edcc34e58ecbb8dbe56d19 \
     --hash=sha256:c70dd71abe5a8c85e55e12c19bd91ccfeec11a6e99044204511f9ed547d48451
-typing==3.6.6 \
-    --hash=sha256:a4c8473ce11a65999c8f59cb093e70686b6c84c98df58c1dae9b3b196089858a \
-    --hash=sha256:57dcf675a99b74d64dacf6fba08fb17cf7e3d5fdff53d4a30ea2a5e7e52543d4 \
-    --hash=sha256:4027c5f6127a6267a435201981ba156de91ad0d1d98e9ddc2aa173453453492d

requirements/codestyle_py2.txt

@@ -0,0 +1,17 @@
+-r codestyle_common.txt
+
+configparser==3.7.1 \
+    --hash=sha256:c114ff90ee2e762db972fa205f02491b1f5cf3ff950decd8542c62970c9bedac \
+    --hash=sha256:df28e045fbff307a28795b18df6ac8662be3219435560ddb068c283afab1ea7a \
+    --hash=sha256:5bd5fa2a491dc3cfe920a3f2a107510d65eceae10e9c6e547b90261a4710df32
+enum34==1.1.6 \
+    --hash=sha256:6bd0f6ad48ec2aa117d3d141940d484deccda84d4fcd884f5c3d93c23ecd8c79 \
+    --hash=sha256:8ad8c4783bf61ded74527bffb48ed9b54166685e4230386a9ed9b1279e2df5b1 \
+    --hash=sha256:2d81cbbe0e73112bdfe6ef8576f2238f2ba27dd0d55752a776c41d38b7da2850
+typing==3.6.6 \
+    --hash=sha256:a4c8473ce11a65999c8f59cb093e70686b6c84c98df58c1dae9b3b196089858a \
+    --hash=sha256:57dcf675a99b74d64dacf6fba08fb17cf7e3d5fdff53d4a30ea2a5e7e52543d4 \
+    --hash=sha256:4027c5f6127a6267a435201981ba156de91ad0d1d98e9ddc2aa173453453492d
+functools32==3.2.3-2 \
+    --hash=sha256:f6253dfbe0538ad2e387bd8fdfd9293c925d63553f5813c4e587745416501e6d \
+    --hash=sha256:89d824aa6c358c421a234d7f9ee0bd75933a67c29588ce50aaa3acdf4d403fa0

requirements/codestyle_py3.txt

@@ -0,0 +1 @@
+-r codestyle_common.txt

tox.ini

@@ -1,5 +1,10 @@
 [tox]
-envlist = es, addons-versions-and-files, amo-lib-locales-and-signin, devhub, reviewers-and-zadmin, accounts-users-and-ratings, main, ui-tests, flake8, docs, assets
+envlist =
+    es, devhub, main, ui-tests, docs, assets, codestyle
+    addons-versions-and-files,
+    amo-lib-locales-and-signin,
+    reviewers-and-zadmin,
+    accounts-users-and-ratings,
 
 [travis]
 unignore_outcomes = True
@@ -85,8 +90,8 @@ commands =
 [testenv:codestyle]
 recreate = True
 commands =
-    pip install -r requirements/flake8.txt
-    make flake8
+    make -f Makefile-docker setup-codestyle
+    make lint-codestyle
 
 [testenv:docs]
 commands =