Update pipeline to check types and formatting

See merge request !27

Explain how to format and type check.

(cherry picked from commit f8217c9d)

(cherry picked from commit 2576afc0)

Drop root privileges in container

See merge request !26

This reverts commit 5dfb6a96.
Because it actually also commits a color change I don't want to have.

Because building it actually means also publishing it, so we only want that when the tests succeed.

Revert "Merge branch 'update-pipeline' into 'master'"

See merge request !25

This reverts merge request !24

Update pipeline to check types and formatting

See merge request !24

Explain how to format and type check.

Make MyPy happy

See merge request !22

db = SQLAlchemy() should be enough to already infer the type of the variable

Update to solve the LSP errors via typing changes and not by code changes.
This undos LSP-violation related code changes.

The argument can only be a string or None, because
dateime.from_isoformat expects only a string.

According to mypy the previous annotation violated the LSP.
We now use the types of the supertype method and return a
datetime object only if the argument was a datetime-object.

According to mypy the previous annotation violated the LSP.
Therefore, we use the types of the supertype method and check for
correct types in the method.