Add bugbear to pre-commit and some code improvements

[juanitorduz]

Bugbear (https://docs.astral.sh/ruff/rules/#flake8-bugbear-b) is a great tool for detecting places where bugs can appear. This PR suggests adding this to the code base. On the way I found many places where we had mutable defaults in functions, which are very dangerous https://florimond.dev/en/posts/2018/08/python-mutable-defaults-are-the-source-of-all-evil

[juanitorduz]

This was a particularly tricky bug as ssc was both a global function and a function parameter.

[codecov]

Codecov Report

Attention: Patch coverage is 78.37838% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyfixest/estimation/feols_.py	55.55%	4 Missing ⚠️
pyfixest/utils/dgps.py	0.00%	4 Missing ⚠️

Flag	Coverage Δ
core-tests	77.80% <78.37%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pyfixest/estimation/FixestMulti_.py	80.20% <100.00%> (ø)
pyfixest/estimation/FormulaParser.py	96.48% <ø> (-0.02%)	⬇️
pyfixest/estimation/estimation.py	96.15% <100.00%> (+0.27%)	⬆️
pyfixest/estimation/feiv_.py	98.16% <100.00%> (ø)
pyfixest/estimation/fepois_.py	87.20% <100.00%> (ø)
pyfixest/report/summarize.py	87.00% <100.00%> (+0.21%)	⬆️
pyfixest/estimation/feols_.py	83.24% <55.55%> (+0.25%)	⬆️
pyfixest/utils/dgps.py	6.30% <0.00%> (-0.24%)	⬇️

[s3alfisc]

Looks great, thank you Juan! I keep on being amazed by the Python tooling that is out there =) Everything looks good based on a first casual glance, though I will take a second look tomorrow before merging (I do not really trust myself after 11:00 😄 ).

[juanitorduz]

Sure! Take a detailed review, as some of the changes were made by the pre-commit hook (no rush to merge).

I'll continue adding more guard rails to make the code even better than it is 💪

[s3alfisc]

Looks all good to me! Looking forward to more guardrails =) Thanks Juan @juanitorduz!