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

docs/admin_api/version_api.md

@@ -16,6 +16,6 @@ It returns a JSON body like the following:
 ```json
 {
     "server_version": "0.99.2rc1 (b=develop, abcdef123)",
-    "python_version": "3.6.8"
+    "python_version": "3.7.8"
 }
 ```