AVRO-2921: Add Type Hints to avro.schema

[kojiromike]

What is the purpose of the change

This is part of the incremental effort to add type checking to the Python implementation.

Verifying this change

This change is already covered by existing tests.

Documentation

This change does not add any new user-facing features. Type annotations may add clearer responses for users who elect to run a type checker. Hopefully those responses are sufficient documentation in themselves.