fix: type-checking Eve dynamic attrs

[raulparada]

Description

Adding event hooks to an Eve instance produces types errors such as:

Cannot assign to attribute "on_insert_<redacted>" for class "Eve"
  Attribute "on_insert_<redacted>" is unknown. Pylance[reportAttributeAccessIssue](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportAttributeAccessIssue)

by pyright/pylance. The pyright maintainer's advice for such cases is to add a __setattr__ to such classes which allow dynamic attributes. Doing that here. Note it's behind the typing.TYPE_CHECKING guard which makes it have no effect at runtime, thus it only impacts type-checking. I've confirmed this fixes the type errors on pyright.

Why care about pyright?

It's the more correct python type-checker (see official python typing benchmarks). Chances are other type-checkers (most importantly mypy) may not fail on this temporarily only due to their lack of coverage.