Bump minimum requirement for requests to 2.22.0
Bump minimum requirement for requests to 2.22.0

Requests 2.22.0 was released on 2019-05-15.

Closes #282 (CRLF injection vulnerability in urllib3).

See psf/requests#5092.

MechanicalSoup/MechanicalSoup,hickford/MechanicalSoup