Import requests in its entirety

Rather than using `from requests import Session`, use `import requests`.
The former may appear to provide a neater interface, but the
recurring_tasks.py script needs to be able to access Requests'
exceptions.HTTPError to be able to handle tasks that have been deleted,
and it makes sense to access that via the habitrpg import rather than
by importing requests directly just for the exceptions.
Merge branch 'bug/requests.exceptions'