Fix CI in newer Python versions

[utensil]

No description provided.

[utensil]

To my surprise, the CircleCI checker is still there, but then I deactivated it in https://github.com/pygae/galgebra/settings/hooks .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.03%. Comparing base (c31d0e4) to head (4c6efe8).
Report is 35 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
- Coverage   78.87%   78.03%   -0.84%     
==========================================
  Files          17       17              
  Lines        4189     4193       +4     
==========================================
- Hits         3304     3272      -32     
- Misses        885      921      +36     

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[utensil]

Notes on local testing various Python versions on my Mac:

brew install pyenv
pyenv install 3.9
pyenv init
source ~/.zprofile
pyenv shell 3.9
pip install -e .
pip install -r test_requirements.txt
pip install jupyter
jupyter notebook
# A smoke test
pytest --nbval examples/ipython/Terminal.ipynb --nbval-current-env --nbval-sanitize-with test/.nbval_sanitize.cfg

[utensil]

All CI green. The code coverage drop is due to adding a conftest.py. Merging.