Improve type of function callbacks in register_upgrade_function and register_downgrade_function

[JeanChristopheMorinPerso]

Link the Issue(s) this Pull Request is related to.

Fixes #1425

Summarize your change.

Improve the advertised types of the callback functions in register_upgrade_function and register_downgrade_function functions.

See https://pybind11.readthedocs.io/en/stable/advanced/cast/functional.html for the Pybind11 documentation of pybind11/functional.h.

The help on both functions previously looked like:

register_downgrade_function(...) method of builtins.PyCapsule instance
    register_downgrade_function(schema_name: str, version_to_downgrade_from: int, downgrade_function: object) -> bool

register_upgrade_function(...) method of builtins.PyCapsule instance
    register_upgrade_function(schema_name: str, version_to_downgrade_from: int, upgrade_function: object) -> bool

while now it looks like:

register_downgrade_function(...) method of builtins.PyCapsule instance
    register_downgrade_function(schema_name: str, version_to_downgrade_from: int, downgrade_function: Callable[[opentimelineio._otio.AnyDictionary], None]) -> bool

register_upgrade_function(...) method of builtins.PyCapsule instance
    register_upgrade_function(schema_name: str, version_to_downgrade_from: int, upgrade_function: Callable[[opentimelineio._otio.AnyDictionary], None]) -> bool

[ssteinbach]

Nice!