Install 3rd party type stubs for mypy

[khaeru]

As part of #327, the mypy step of the lint workflow began to fail.

This is because mypy 0.900 et seq. were released; in these versions (unlike mypy 0.8xx), type stubs for 3rd-party packages are not bundled and must be installed separately.

This PR adds these to lint.yaml:

• types-requests
• types-PyYAML
• types-pkg_resources

How to review

No review/FYI only: changes to CI.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A, CI only
• Update release notes.

[codecov]

Codecov Report

Merging #484 (18faabf) into main (d52370b) will not change coverage.
The diff coverage is n/a.

❗ Current head 18faabf differs from pull request most recent head d1d6166. Consider uploading reports for the commit d1d6166 to get more accurate results

@@          Coverage Diff          @@
##            main    #484   +/-   ##
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  Coverage   93.3%   93.3%           
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  Files         37      37           
  Lines       2819    2819           
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  Hits        2632    2632           
  Misses       187     187