Update dependency slotscheck to v0.21.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
slotscheck	==0.20.1 → ==0.21.0

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Release Notes

ariebovenberg/slotscheck (slotscheck)

v0.21.0

Compare Source

• Add --pythonpath option to add directories to Python's import path.
  This is equivalent to setting $PYTHONPATH, but usable in places where
  environment variables are awkward to set (such as pre-commit on Windows).
• When a file isn't importable, the error message now suggests the directory
  to add to the import path, instead of only linking to the documentation.

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