Use the responses library rather than httpd

[edmorley]

TestAPI() currently uses httpd to fake out Treeherder API responses, when this could be done much more simply using the responses library:
https://github.com/getsentry/responses

This would also mean the host and protocol arguments to Treeherder could be dropped, since all other consumers of it use the default Treeherder production instance.

[whimboo]

Sweet. Thanks for this hint @edmorley. I haven't known this package yet. It indeed sounds like a good idea.