[securedrop-proxy] Consider using requests module

[kushaldas]

Currently we are building HTTP request objects manually in the codebase, and then using that to do HTTP calls. We should consider using Python requests module instead as it the most consumed Python module outside of the standard library.

[sssoleileraaa]

[Backlog pruning, 5/16] Endpoint specific code could be generated in the SDK, but since this is a smaller issue and helps with standardization in the meantime (the endpoint refactor hasn't been planned for anytime soon) we're keeping this open.

[legoktm]

The proxy has been using the requests module for quite a while now.