Remove python 3.6 support

cryptography raises a warning on python 3.6, so we may as well follow suit.

.taskcluster.yml

@@ -84,8 +84,6 @@ tasks:
           $map:
             - short_name: py27
               image_tag: '2.7'
-            - short_name: py36
-              image_tag: '3.6'
             - short_name: py37
               image_tag: '3.7'
             - short_name: py38

maintenance/pin.sh

@@ -4,4 +4,4 @@ set -e
 set -x
 
 docker run -t -v $PWD:/src -w /src -e SUFFIX=-py2 python:2.7 maintenance/pin-helper.sh
-docker run -t -v $PWD:/src -w /src python:3.6 maintenance/pin-helper.sh
+docker run -t -v $PWD:/src -w /src python:3.7 maintenance/pin-helper.sh

setup.py

@@ -67,7 +67,6 @@ setup(
         'Programming Language :: Python',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: Implementation :: CPython',

tox.ini

@@ -4,7 +4,7 @@
 envlist =
     clean,
     check,
-    {py27,py36,py37,py38,py39,py310,pypy},
+    {py27,py37,py38,py39,py310,pypy},
     docs,
     report
 
@@ -12,7 +12,6 @@ envlist =
 basepython =
     pypy: {env:TOXPYTHON:pypy}
     py27: {env:TOXPYTHON:python2.7}
-    py36: {env:TOXPYTHON:python3.6}
     py37: {env:TOXPYTHON:python3.7}
     py38: {env:TOXPYTHON:python3.8}
     py39: {env:TOXPYTHON:python3.9}