add missing escape character

[Gautzilla]

Should resolve the syntax warning when using the strptime function.

[Gautzilla]

I struggled a tiny bit an awful lot with the handling of warnings in pytest.

At first I wanted to add an assert to the tests that check that no SyntaxWarning is raised anymore, but I couldn't figure out a simple way to do so.

I ended up turning SyntaxWarnings into errors in the pytest scope (by adding a filter to the pyproject.toml file): That way, the tests would just throw an error if a SyntaxWarning is encountered.

This happens project-wide tho, so we'll have to be a bit meticulous on the syntax (I guess that's kind of a good thing 🥸)