Update infrastructure

[JoshKarpel]

• Drop support for Python 3.7
• Switch to Poetry for package management
• Update pre-commit hooks and GitHub Actions
• Switch docs to mkdocs

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@bc2a168). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head 9c9404f differs from pull request most recent head 8a74295. Consider uploading reports for the commit 8a74295 to get more accurate results

Additional details and impacted files

@@          Coverage Diff           @@
##             main     #88   +/-   ##
======================================
  Coverage        ?   98.3%           
======================================
  Files           ?      11           
  Lines           ?     306           
  Branches        ?      77           
======================================
  Hits            ?     301           
  Misses          ?       2           
  Partials        ?       3           

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