fix: drop usage of distutils

[parthea]

Fixes #507 🦕

• Raises ValueError if the format of the GOOGLE_API_USE_CLIENT_CERTIFICATE is not as defined below
  

  python-api-core/google/api_core/operations_v1/abstract_operations_client.py

  Lines 275 to 280 in 405272c

  	(2) If GOOGLE_API_USE_CLIENT_CERTIFICATE environment variable
  	is "true", then the ``client_cert_source`` property can be used
  	to provide client certificate for mutual TLS transport. If
  	not provided, the default SSL client certificate will be used if
  	present. If GOOGLE_API_USE_CLIENT_CERTIFICATE is "false" or not
  	set, no client certificate will be used.

This is the same behaviour that exists in gapic-generator-python here https://github.com/googleapis/gapic-generator-python/blob/45f18a69454bcff68c0f98535c68fbdb4e09db1d/gapic/templates/%25namespace/%25name_%25version/%25sub/services/%25service/client.py.j2#L253-L256

[vchudnov-g]

Only a minor, non-blocking comment.

[vchudnov-g]

Might as well us lower() in this assignment so you don't have to repeat it below

(It would be great to cast this to a bool like the previous code was doing, but it seems like too much overhead for the very limited use in a very compact LOC range. The string comparison is OK.)

[parthea]

Fixed in 614150d