chore: drop support for python 3.6

It's been EOL since 23 Dec 2021 and is now no longer passing tests.

If someone does still use this library with python 3.6, they will need
to pin the `bleach` dependency to 4.1.0 instead - this library ought to
still work, but no guarantees.

Signed-off-by: Mike Fiedler <miketheman@gmail.com>

setup.py

@@ -48,7 +48,6 @@
         "Programming Language :: Python",
         "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3 :: Only",
-        "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py37,py38,py39,py310,pep8,packaging,noextra,mypy
+envlist = py37,py38,py39,py310,pep8,packaging,noextra,mypy
 isolated_build = True
 
 [testenv]