Use a fresh connection on every request

[grego118]

We shouldn't be using a shared requests.Session() object across all requests that a client makes. If the client (and therefore the session) is shared across multiple threads or greenlets we can encounter a race condition that causes the following error:

('Connection aborted.', RemoteDisconnected('Remote end closed connection without response'))

This happens because the session maintains a connection pool for each host (keep-alive). If a connection is closed by the server around the same time a thread tries to make a new request using that connection, then the new request will be made on the already closed connection.

This PR simply removes the session member and lets the requests package make a new connection on every request.