Pin urllib3 in global constraints

[mumarkhan999]

Description

The new version of urllib3 isn't compatible with httppretty and causing the following errors

E           httpretty.errors.UnmockedError: Failed to socket.shutdown because because a real socket does not exist
E           
E           Tip: You could try setting (allow_net_connect=True) to allow unregistered requests through a real TCP connection in addition to (verbose=True) to debug the issue.

More details: gabrielfalcao/HTTPretty#484