Update mypy presubmits for generated clients to use `--strict` option

[parthea]

Mypy has a --strict option. We should update the mypy tests for generated clients from mypy -p google to mypy -p --strict google

https://github.com/googleapis/gapic-generator-python/blob/1b63310378692f478ef29734b3cb5fde3d436b22/gapic/templates/noxfile.py.j2#L186-L194

There is some guidance here for setting stricter options.

An excellent goal to aim for is to have your codebase pass when run against mypy --strict. This basically ensures that you will never have a type related error without an explicit circumvention somewhere (such as a # type: ignore comment).

[parthea]

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