add setuptools to dependencies

[cbini]

If you import this library in an isolated environment, it will fail with ModuleNotFoundError: No module named 'pkg_resources' due to the line referenced below. While setuptools is a commonly installed alongside Python, it's not a core module, and should be listed as a dependency so that package managers (e.g. pipenv, poetry, pdm) will pick up on it and install it.

python-storage/google/cloud/storage/_http.py

Line 19 in a8fcebb

import pkg_resources

[andrewsg]

You're right! Our automated testing uses setuptools to install the dependencies so it's always installed, but that's not necessarily true of other environments. Thank you for letting us know.