fix: Add deprecation warning for python 3.7

[parthea]

Towards googleapis/google-cloud-python#11437

Using Python 3.7

(py3716) partheniou@partheniou-vm-3:~/git/gapic-generator-python/tests/integration/goldens/credentials$ export PYTHONWARNINGS="default"
(py3716) partheniou@partheniou-vm-3:~/git/gapic-generator-python/tests/integration/goldens/credentials$ python3.7
Python 3.7.16 (default, Mar 14 2023, 12:34:43) 
[GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import google.iam.credentials_v1
/usr/local/google/home/partheniou/git/gapic-generator-python/tests/integration/goldens/credentials/google/iam/credentials_v1/__init__.py:54: Python37DeprecationWarning: After January 1, 2024, new releases of this client library will drop support for python 3.7. More details about Python 3.7 support for Client Libraries can be found at https://cloud.google.com/python/docs/python37-sunset/
  warnings.warn(message, Python37DeprecationWarning)
>>> import google.iam.credentials
/usr/local/google/home/partheniou/git/gapic-generator-python/tests/integration/goldens/credentials/google/iam/credentials/__init__.py:54: Python37DeprecationWarning: After January 1, 2024, new releases of this client library will drop support for python 3.7. More details about Python 3.7 support for Client Libraries can be found at https://cloud.google.com/python/docs/python37-sunset/
  warnings.warn(message, Python37DeprecationWarning)
>>> import google.iam.credentials_v1
>>> import google.iam.credentials
>>> 

Using Python 3.9

(py39) partheniou@partheniou-vm-3:~/git/gapic-generator-python/tests/integration/goldens/credentials$ export PYTHONWARNINGS="default"
(py39) partheniou@partheniou-vm-3:~/git/gapic-generator-python/tests/integration/goldens/credentials$ python3.9
Python 3.9.16 (main, Mar  2 2023, 17:52:22) 
[GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import google.iam.credentials_v1
/usr/local/google/home/partheniou/.pyenv/versions/py39/lib/python3.9/site-packages/google/rpc/__init__.py:18: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
  import pkg_resources
...
>>> import google.iam.credentials
>>> import google.iam.credentials_v1

The warnings that appear with python 3.9 are captured in googleapis/python-api-common-protos#140 (comment) and googleapis/google-cloud-python#11184

[parthea]

Suggested change

	# Configure the Python37DeprecationWarning warning so that it is only emitted once.
	warnings.simplefilter('once', Python37DeprecationWarning)

This code should be removed. See googleapis/google-auth-library-python#1427

[parthea]

Suggested change

	# print only the first occurrence of Python37DeprecationWarning, regardless of location
	warnings.simplefilter('once', Python37DeprecationWarning)

This code should be removed. See googleapis/google-auth-library-python#1427