Update Python requirements with

    (cd requirements; rm *.txt; make)

This updates Django from version 3.1 to version 3.1.7.
This updates Wagtail from version 2.10 to version 2.12.3

The virtual environment can be update to the specified versions with

    pip-sync requirements/dev.txt

After that it might be necessary to reply the patches

   cd VENV
   for f in SRC/patches/* ; do patch -p1 <$f ; done

Also it might be necessary to run migrations

   ./manage.py migrate

and collect status files

   ./manager.py collectstatic

Run

    ./manage.py makemessages -l de -i xlwt

It might be necessary to run

    ./manage.py compilemessages -l de

Fix import after Wagtail update