feat: add Python 3.10 support

[alexander-held]

Since pyhf now supports 3.10 (scikit-hep/pyhf#1808, scikit-hep/pyhf#1809), cabinetry should also officially support it. This adds the relevant metadata and adds Python 3.10 to the CI. See #277 for the corresponding PR that added Python 3.9 support.

This should only be merged after #301, which adds support for the upcoming pyhf 0.7 release.

* add Python 3.10 support and test Python 3.10 in CI
* updated pre-commit

[codecov]

Codecov Report

Merging #334 (1c54744) into master (ba834d2) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #334   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1876      1876           
  Branches       298       298           
=========================================
  Hits          1876      1876           

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