Failure with Python 3.9 due to use of deprecated collections.Mapping

[jay0lee]

Environment details

• OS: Linux
• Python version: 3.9 alpha 2
• pip version: 19.3.1
• google-auth version: 1.10.0

Steps to reproduce

1. Use Python 3.9
2. Attempt to decode an ID Token:
   google.oauth2.id_token.verify_oauth2_token()...

Expected results:
decoded ID token

Actual results:
AttributeError: module 'collections' has no attribute 'Mapping'
see full crash at:
AttributeError: module 'collections' has no attribute 'Mapping'

Further details:
collections.Mapping was moved to collections.abc.Mapping in Python 3.3 but mapped back to collections up to Python 3.8. In Python 3.9 the legacy mapping is removed so we break. See:
https://docs.python.org/3/library/collections.html#