Remove Python 3.7 support; switch from slow and deprecated pkg_resources to importlib.metadata

[andersk]

Python 3.7 reached end of life on June 27, 2023: https://devguide.python.org/versions/

Fixes this warning in Python development mode:

…/formtools/__init__.py:2: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
  from pkg_resources import DistributionNotFound, get_distribution

[codecov[bot]]

Codecov Report

Merging #242 (5502804) into master (0e60f1d) will not change coverage. The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #242   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files          11       11           
  Lines         535      535           
  Branches       68       68           
=======================================
  Hits          502      502           
  Misses         21       21           
  Partials       12       12           

Files Changed	Coverage Δ
formtools/__init__.py	50.00% <66.66%> (ø)

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