Constrain Requests to < 2.13.0 if on setuptools < 18.0.0
Constrain Requests to < 2.13.0 if on setuptools < 18.0.0

See #307 for context, but the Requests packaged added a release that
broke a lot of people on old versions of pip/setuptools. Here we
constrain the library to < 2.13.0 if we detect setuptools < 18.0.0.

Fixes #307.

stripe/stripe-python