tests: Exclude mypy-specific code from coverage

[l0b0]

Related Issue(s): #331

Description:

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #492 (1fd4c4b) into main (f9573f8) will increase coverage by 0.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
+ Coverage   91.66%   92.10%   +0.43%     
==========================================
  Files          40       40              
  Lines        5245     5206      -39     
==========================================
- Hits         4808     4795      -13     
+ Misses        437      411      -26     

Impacted Files	Coverage Δ
pystac/collection.py	95.88% <0.00%> (+0.37%)	⬆️
pystac/stac_object.py	95.18% <0.00%> (+0.47%)	⬆️
pystac/catalog.py	95.21% <0.00%> (+0.48%)	⬆️
pystac/serialization/identify.py	93.65% <0.00%> (+0.68%)	⬆️
pystac/summaries.py	79.73% <0.00%> (+0.89%)	⬆️
pystac/serialization/migrate.py	88.52% <0.00%> (+1.22%)	⬆️
pystac/extensions/hooks.py	87.71% <0.00%> (+1.27%)	⬆️
pystac/stac_io.py	84.11% <0.00%> (+1.35%)	⬆️
pystac/cache.py	95.72% <0.00%> (+1.55%)	⬆️
pystac/layout.py	94.97% <0.00%> (+2.03%)	⬆️
... and 3 more

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[l0b0]

This needs to be re-run because of a flaky test.