Remove support for Python 3.7
Add Python 3.8, 3.9 and 3.10 to unit tests runs.

Instructions to run unit tests:

1. Without Docker:
  * Make sure you have pyenv installed (go/adr-python).
  * Make sure Python versions 3.8, 3.9 and 3.10 are installed.
  * From project root run: pyenv local 3.8 3.9 3.10
  * From project root run: nox
2. With Docker:
  * Make sure you have Docker installed.
  * From project root run: docker build -t google-ads-python:test .
  * From project root run: docker run -it google-ads-python:test cd nox

Change-Id: I64b1fa622143dced50c690f69410e84f7b7d27bf

Python 3.11 and 3.12.

Change-Id: Ie33c74a1f74418550d49e74670c6328eea4279d8

  * Removes google/__init__.py and google/ads/__init__.py
  * Switch to find_namespace_packages in setup.py
2. Remove usage of pkg_resources
  * Regenerate v13, v14, and v15 stubs with gapic generator v1.13.0
  * Remove from googleads/client.py and rely on default transport
    parameter
  * Remove from metadata interceptor and reply on __version__ for pb
    version.

Change-Id: I7fec326731ac321be131270843aba6cf5d76cff8

example.