BLD: add importlib-metadata dependency for python < 3.10

[mj-will]

When adding support for entry points, I forgot to add importlib-metadata as an explicit dependency for Python < 3.10.
This PR adds that dependency.

Once this is merged, I'll backport this to v0.14.0 and make a post-release.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.89%. Comparing base (17db788) to head (5da3e50).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #450   +/-   ##
=======================================
  Coverage   93.89%   93.89%           
=======================================
  Files          72       72           
  Lines        7027     7027           
=======================================
  Hits         6598     6598           
  Misses        429      429           

Flag	Coverage Δ
unittests	93.89% <ø> (ø)

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