Switch license badge to one that reads from GitHub

[stefmolin]

After the changes to the metadata necessary for publishing with setuptools, it looks like the old badge isn't finding the license on PyPI (despite it being on there). This PR switches it to use license file from GitHub.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.80%. Comparing base (a84464e) to head (d08f11b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #308   +/-   ##
=======================================
  Coverage   98.80%   98.80%           
=======================================
  Files          84       84           
  Lines        2099     2099           
  Branches      123      123           
=======================================
  Hits         2074     2074           
  Misses         24       24           
  Partials        1        1           

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