Add support for Python 3.8

[felixonmars]

Fixes errors in the new Python release:

  File "/build/python-braintree/src/braintree_python-3.57.1/braintree/transparent_redirect_gateway.py", line 52, in _parse_and_validate_query_string
    query_params = cgi.parse_qs(query_string)
AttributeError: module 'cgi' has no attribute 'parse_qs'

Summary

Checklist

• [ ] Added changelog entry
• [ ] Ran unit tests (nosetests tests/unit)

[crookedneighbor]

Sorry for the long delay, this looks great. I ran the integration tests against python 3.7 and 3.8 and everything looks :+1:

[crookedneighbor]

This is out in 3.58.0. Thanks for your work on this!