Drop support for and remove references to EOL Python 3.6 (#11683)

* remove reference in comments to python3.6

* upgrade tox python env in script

* bump python version in example for completeness

* upgrade python version requirement in setup doc

* upgrade necessary python version in __init__.py

* upgrade python version in setup.py

* newsfragment

* drops refs to bionic and replace with focal

* bump refs to postgres 9.6 to 10

* fix hanging ci

* try installing tzdata first

* revert change made in b979f336

* ignore new random mypy error while debugging other error

* fix lint error for temporary workaround

* revert change to install list

* try passing env var

* export debian frontend var?

* move line and add comment

* bump pillow dependency

* bump lxml depenency

* install libjpeg-dev for pillow

* bump automat version to one compatible with py3.8

* add libwebp for pillow

* bump twisted trunk python version

* change suffix of newsfragment

* remove redundant python 3.7 checks

* lint

setup.py

@@ -150,7 +150,7 @@ setup(
     zip_safe=False,
     long_description=long_description,
     long_description_content_type="text/x-rst",
-    python_requires="~=3.6",
+    python_requires="~=3.7",
     entry_points={
         "console_scripts": [
             "synapse_homeserver = synapse.app.homeserver:main",