Migrate from pkg_resources to importlib.metadata

[mcmtroffaes]

Resolves issue #298.

[codecov]

Codecov Report

Merging #299 (c87a56f) into develop (0c496c1) will not change coverage.
The diff coverage is 100.00%.

❗ Current head c87a56f differs from pull request most recent head ec622c4. Consider uploading reports for the commit ec622c4 to get more accurate results

@@            Coverage Diff            @@
##           develop      #299   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           52        52           
  Lines         2428      2435    +7     
  Branches       425       427    +2     
=========================================
+ Hits          2428      2435    +7     

Impacted Files	Coverage Δ
src/sphinxcontrib/bibtex/plugin.py	100.00% <100.00%> (ø)
src/sphinxcontrib/bibtex/style/template.py	100.00% <100.00%> (ø)

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