This kind of diagnostic:

/home/runner/work/githubkit/githubkit/githubkit/versions/v2022_11_28/webhooks/pull_request.py:67:17 - error: Argument of type "UnionType" cannot be assigned to parameter "type_" of type "type[T@TaggedUnion]" in function "__init__"
  Type "UnionType" is not assignable to type "type[T@TaggedUnion]" (reportArgumentType)
Error: Argument of type "UnionType" cannot be assigned to parameter "type_" of type "type[T@TaggedUnion]" in function "__init__"

can be solved with a # type: ignore (probably preferred, this way it will work at least for "regular" classes passed), a tricky extra overload or Any. The future solution for exactly this kind of problem is TypeForm (PEP-741), but it isn't officially approved and stabilized yet.

Pyright has experimental support for TypeForm (with enableExperimentalFeatures), TypeForm can be imported from typing_extensions. But beware that

enableExperimentalFeatures [boolean]: [...] These features will likely change or be removed, so they should not be used except for experimentation purposes.