Don't change matplotlib style on import of causalpy + misc dev fixes

[drbenvincent]

• Closes Importing causalpy changes matplotlib style #519 so will not longer change the matplotlib style when causalpy is imported.
• Made a few other small fixes to the Makefile to resolve some issues I was having.
• Also had to pin Python version to >=3.11 to avoid environment problems.

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📚 Documentation preview 📚: https://causalpy--538.org.readthedocs.build/en/538/

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.48%. Comparing base (edb7f8d) to head (edcb861).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
causalpy/experiments/base.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #538      +/-   ##
==========================================
+ Coverage   95.43%   95.48%   +0.05%     
==========================================
  Files          28       29       +1     
  Lines        2586     2615      +29     
==========================================
+ Hits         2468     2497      +29     
  Misses        118      118              

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[williambdean]

Small comment on location of import but not a blocker