Swap rate-limited PyPI downloads badge for pepy total downloads

[ronpandolfi]

The shields.io PyPI /dm/ endpoint has been getting rate-limited; pepy serves cumulative download counts from a separate backend.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.33%. Comparing base (f9fc76f) to head (a77770e).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   87.52%   88.33%   +0.81%     
==========================================
  Files           9        9              
  Lines         617      617              
==========================================
+ Hits          540      545       +5     
+ Misses         77       72       -5     

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