Introduce deprecation warning for python 3.7, google-api-core 1.x and protobuf 3.x

[holtskinner]

Python 3.7 is now unsupported in the python community. We should update the README to indicate that python 3.7 support will be dropped and also introduce a deprecation warning with the same information.

Similar to:

• Introduce deprecation warning for python 3.7, google-api-core 1.x and protobuf 3.x google-cloud-python#11437
• chore: Add deprecation notice for 3.7. google-auth-library-python#1371

[holtskinner]

Note: When Python 3.7 support is dropped, this can be removed from all tests:

# try/except added for compatibility with python < 3.8
try:
    from unittest import mock
except ImportError:  # pragma: NO COVER
    import mock

Change to:

from unittest import mock