fix: compat with python 3.8 by removing beartype

[MilesCranmer]

beartype seems to have some builds on conda-forge that are incompatible with older python. These can't be removed without significant effort. But at the end of the day, we were only using the beartype import for one single type (typing.List, since Python will eventually deprecate this). Therefore, in order to keep compatibility with old versions, I need to remove beartype as a mandatory requirement. It turns out that typing_extensions also has a List so I'll just do that.

Regardless, beartype is still used during testing.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.54%. Comparing base (03429bc) to head (3bd5fc8).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
pysr/sr.py	50.00%	2 Missing ⚠️
pysr/utils.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
- Coverage   93.77%   93.54%   -0.24%     
==========================================
  Files          21       21              
  Lines        1559     1565       +6     
==========================================
+ Hits         1462     1464       +2     
- Misses         97      101       +4     

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