Update MyPy to 2.1.0, with necessary fixes

[jholveck]

MyPy 2.1.0 found some places where the type changes from BoboTiG#521 weren't correctly propagated.

This should supercede PR BoboTiG#524 (dependabot's bump).

Changes proposed in this PR

Fixes #
(...)

• Tests added/updated
• Documentation updated
• Changelog entry added
• ./check.sh passed

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