Explicitly depend on setuptools

[lukesneeringer]

Context: #3295

We should explicitly depend on a minimum version of setuptools to get around problems installing our packages if setuptools is too old.

[dhermes]

This is just like a CYA? Isn't this kind of the chicken or the egg, like setup.py doesn't work because of an old version of setuptools, so we are going to require a newer version in setup.py?

[ndenny]

I've hit this problem in the past few days - google-cloud-* modules can't be installed on clean Python environments installed using PyEnv, so had to use pip to upgrade setuptools first. Seems reasonable that you add a include the requirement so you can be installed seamlessly: it's not an obvious error, and quite hard to find this issue to find the answer.

[dhermes]

@ndenny I've had the same issue with "fresh-ish" pyenv environments, thanks for pointing this out.