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

gapic-generator-python/gapic/templates/noxfile.py.j2

Lines 186 to 194 in 1b63310

	session.run(
	'mypy',
	'-p',
	{% if api.naming.module_namespace %}
	'{{ api.naming.module_namespace[0] }}',
	{% else %}
	'{{ api.naming.versioned_module_name }}',
	{% endif %}
	)

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).