Upgrade django and other python dependencies

Upgrade django to 4.1
Upgrade django-timezone-field, django-simple-history, django-debug-toolbar and django-bootstrap-datepicker-plus
Upgrade mysqlclient
Remove obsolete pytz
Adapt timezone handling code to new interface of TimezoneInfo (part of python since 3.9 and backport included in 3.8)
Drop support for python 3.7 (since Django 4 requires >= 3.8)