Migrate to pyproject.toml instead of setup.py

[parthea]

pyproject.toml is recommended instead of setup.py which is currently in the generated output for GAPIC.

Follow this tutorial to use pyproject.toml
https://packaging.python.org/en/latest/tutorials/packaging-projects/#creating-pyproject-toml

The sample project on PyPA was recently updated to migrate from setup.py/setup.cfg to pyproject.toml
pypa/sampleproject#166

[chalmerlowe]

This may not be an issue if we are using a recent version of pip (21.3+ I think...)

At one point even if you switched to pyproject.toml there was a requirement to provide a shim in setup.py if you wanted to be able to install a package in editable mode (i.e. using pip install -e):

#!/usr/bin/env python

import setuptools

if __name__ == "__main__":
    setuptools.setup()

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.