Switch to use `pyproject.toml` over `setup.py`

Switch to use `pyproject.toml` for package management as `setup.py` is a thing of the past.

Ref: https://packaging.python.org/en/latest/guides/writing-pyproject-toml/

Cloud SQL PR is up: https://github.com/GoogleCloudPlatform/cloud-sql-python-connector/pull/1230