Remove setuptools from the install_requires

The code itself is a no-op as the setup.py will not even compile without
setuptools pre-existing. However, there are situations, specifically
related to pip install -U cryptography, or even worse pip install -U
something_that_depends_transitively_on_cryptography where certain
versions of setuptools and distribute can interact in ways as to make a
user's python install broken. Advice from the pypa folks is to never
directly depend on setuptools.