Add version to `requests` in setup.py

[pconerly]

Somewhere between requests==1.2 and requests==2.7, requests switching to using TLS, which makes it work with the prismic cdn. You guys should find what version works and add it to your setup.py. https://github.com/prismicio/python-kit/blob/686dc9a2df72f457838c72ea75c7d58bd0f856de/setup.py#L28

Testing the versions of the other dependencies would also be a good idea.