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

docker/Dockerfile-pgtests

@@ -1,6 +1,6 @@
 # Use the Sytest image that comes with a lot of the build dependencies
 # pre-installed
-FROM matrixdotorg/sytest:bionic
+FROM matrixdotorg/sytest:focal
 
 # The Sytest image doesn't come with python, so install that
 RUN apt-get update && apt-get -qq install -y python3 python3-dev python3-pip

docker/run_pg_tests.sh

@@ -16,4 +16,4 @@ sudo -u postgres /usr/lib/postgresql/10/bin/pg_ctl -w -D /var/lib/postgresql/dat
 # Run the tests
 cd /src
 export TRIAL_FLAGS="-j 4"
-tox --workdir=./.tox-pg-container -e py36-postgres "$@"
+tox --workdir=./.tox-pg-container -e py37-postgres "$@"