python deprecation warning for GetPrototype should be a python DeprecationWarning not a python UserWarning

[matanox]

Minor issue.

protobuf 4.25.5, raises a python warning:

/lib/python3.11/site-packages/google/protobuf/symbol_database.py:55: UserWarning:
  
SymbolDatabase.GetPrototype() is deprecated. Please use message_factory.GetMessageClass() instead. SymbolDatabase.GetPrototype() will be removed soon.

since this is a warning about deprecation, why not make it the python warning type dedicated to deprecations, which is called DeprecationWarning? why does it categorize itself as a UserWarning python warning type?

this can help in some scenario where we don't want to see deprecation warnings in our test runner summary.