refactor: drop support for Python 3.6

BREAKING CHANGE: bleach 5 doesn't support Python 3.6

chore: upgrade filebrowser_safe and grappelli_safe to version 1.1.1

fix: don't enforce uniqueness of empty emails. Fixes #1984

fix: use MiddlewareMixin consistently. Fixes #2019

fix: don't override MySQL test DB collation if set (#2017)

chore: pin pytz version

chore: pin filebrowser and grappelli

chore: pin filebrowser and grappelli versions to RC

ci: configure semantic-release

Version 4.3.1