Define and use a `TypeVar` for `Event` classes

lunkwill42 commented 3 months ago

Scope and purpose

Fix slightly incorrect type annotations for event registry functionality.

This is the canonical way to annotate that the return value of a function is an instance of the type that was used as one of the function arguments.

Seems to be the only way to make type checkers understand the real type of the return argument.

Updates type annotations of the Events class, making IDE type inferences more useful.
Was extracted from #244

[ ] Added a changelog fragment for towncrier
- Changes no outward functionality, only helps devs
[ ] Added/amended tests for new/changed code
- Not necessary, no functionality changed.
[X] Added/changed documentation
- This is, by definition, documentation
[X] Linted/formatted the code with black, ruff and isort, easiest by using pre-commit
[X] The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
[X] If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done