Fix flake8 dependencies, apparently it still needs pycodestyle.

requirements/flake8.txt

@@ -1,4 +1,12 @@
 
+# configparser is required by flake8
+configparser==3.5.0 \
+    --hash=sha256:5308b47021bc2340965c371f0f058cc6971a04502638d4244225c49d80db273a
+# 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==3.0.4 \
     --hash=sha256:603a3ae7c8030219fee084728ca02a8bbd3a51829cacf97b445172a46cb04662 \
@@ -9,6 +17,10 @@ flake8-mutable==1.0.5 \
 mccabe==0.5.2 \
     --hash=sha256:91cc38b2c7636aaf1903e06d96ee960fb3dff9ca3afc595627c9a638f8e86d2b \
     --hash=sha256:3473f06c8b757bbb5cdf295099bf64032e5f7d6fe0ec2f97ee9b23cb0a435aff
+# pycodestyle is required by flake8
+pycodestyle==2.0.0 \
+    --hash=sha256:2ce83f2046f5ab85c652ceceddfbde7a64a909900989b4b43e92b10b743d0ce5 \
+    --hash=sha256:37f0420b14630b0eaaf452978f3a6ea4816d787c3e6dcbba6fb255030adae2e7
 # pyflakes is required by flake8
 pyflakes==1.3.0 \
     --hash=sha256:ad89dafee8ca32282116209a0ca4dff050bdc343af958721d5517d242c1215d5 \