[#1819] Upgrade Requests to 2.3.x

Only a minor change in the resource proxy extension (look like the
content-length header is no longer added by default).
[#1714] do not remove session when get_site_user is called

Only commit if the site user is created. Change clean_db so that it
commits and removes the session so that any open transactions that might
cause clean_db to hang are closed before dropping the tables.
Merge pull request #1714 from ckan/1714-fix-get-site-user

`get_site_user` performs a commit on each call
Merge branch '1819-upgrade-requests'