docs(backport): Use importlib.metadata to get version info

[meeseeksmachine]

Backport PR #2477: docs: Use importlib.metadata to get version info

* Backport PR https://github.com/scikit-hep/pyhf/pull/2477
* pkg_resources is deprecated as an API, so stop using it.
   - c.f. https://setuptools.pypa.io/en/latest/pkg_resources.html
* importlib.metadata.version is the recommended way to get package
  version information.
   - c.f. https://docs.python.org/3/library/importlib.metadata.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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@@                Coverage Diff                @@
##             release/v0.7.x    #2502   +/-   ##
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  Coverage                  ?   98.19%           
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  Files                     ?       69           
  Lines                     ?     4547           
  Branches                  ?      804           
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  Hits                      ?     4465           
  Misses                    ?       49           
  Partials                  ?       33           

Flag	Coverage Δ
contrib	97.73% <ø> (?)
doctest	98.02% <ø> (?)
unittests-3.10	96.19% <ø> (?)
unittests-3.11	96.19% <ø> (?)
unittests-3.12	96.19% <ø> (?)
unittests-3.7	96.17% <ø> (?)
unittests-3.8	96.21% <ø> (?)
unittests-3.9	96.23% <ø> (?)

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