add dependency urllib3

[oxidworks]

fix #49

[gtaylor]

If I am understanding correctly, the fix is to avoid a specific version (1.22), not limit us to a tiny subset of versions. If that is indeed the case, how about something like:

urllib3!=1.22

[oxidworks]

Hello, thx for you help and your work. I get an RuntimeError: Requests dependency 'urllib3' must be version >= 1.21.1, < 1.22! Is there a special meaning for "!" in this case, something other then attention?
Maybe it is more a problem with "requests" on my pc. My first merge request here :-)

[gtaylor]

I think I may actually punt on this, since it's more of a request/urllib3 issue than something this module should be concerned with. It may be best to handle this in whatever package you are writing that uses paypal-python.

I hope this sounds reasonable!