Clean dependencies; add extras feature

[getzze]

Summary:

• reorder pyproject.toml entries to make it easier to find the information.
• add python 3.12 classifier
• remove codecov, openpyxl and mpmath test dependencies as they are not used in tests.
• add extras features to optionally install the mpmath dependency with pip install pingouin[extras].

On a side note, mpmath is an optional dependency as it is checked if it's installed in utils.py.
It's also checked if scikit-learn and statsmodels are installed in utils.py, whereas they are part of the hard dependencies. Meaning they should be installed, at least if installing with pip. These two dependencies could be moved to extras dependencies or the checks in utils.py could be removed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (873eb4b) to head (e22ea7c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #451   +/-   ##
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  Coverage   98.54%   98.54%           
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  Files          19       19           
  Lines        3360     3360           
  Branches      492      492           
=======================================
  Hits         3311     3311           
  Misses         26       26           
  Partials       23       23           

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[raphaelvallat]

Thank you — this is great. I prefer to keep statsmodels and scikit-learn as hard dependencies. I'll merge the PR now.