Add pytest-notebook dependency

[teutoburg]

Doesn't do anything yet, but I'm trying to replace our rather cumbersome way of running the notebooks as part of the tests. This has the advantage that the notebooks can count towards coverage (see also #240, which this does not close yet, but might enable something in the future). I'm still testing what's best for our setup, there are several options. But this seems to work and the dependencies it adds indirectly are not too bad. It does include gitpython (although I'm not quite sure why, but it can do regression testing as well, which might be the reason), which is fine, because I wanted to use that anyway to deal with IRDB packages. Oh and the dependencies are of course only in the tests group, so not required to pip install.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.91%. Comparing base (ec8ed7f) to head (28c1a6f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #976   +/-   ##
=======================================
  Coverage   75.91%   75.91%           
=======================================
  Files          70       70           
  Lines        9037     9037           
=======================================
  Hits         6860     6860           
  Misses       2177     2177           

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