Remove testing dependency on requests

[Jaymon]

Datatypes's HTTPClient has most of the features, including making the basic request header. I think it's only missing the functionality to upload files found in endpoints.client.HTTPClient.post_file, if I add that to datatypes's client then I think I can remove the dependency on requests with very little work since Datatypes's client is designed to be a simple drop in replacement.

[Jaymon]

If I wanted to switchover to an async like requests library, here are some options:

• niquests (pypi)