List treeherder-client v2.0.1 as an explicit dependency
List treeherder-client v2.0.1 as an explicit dependency

treeherder-client is not listed in the requirements file directly, even
though Alert Manager uses it. (It's installed by chance via mozci's
dependency on treeherder-client.)

To both more reliably ensure treeherder-client is installed, and to
update any installed version to the latest, treeherder-client and its
sub-dependencies have been added to the requirements file.

treeherder-client also requires requests >= 2.4.3 (since it uses the
json POST parameter), so requests has also been updated to the latest
release (keeping it up to date is always recommended, since it bundles
newer versions of certifi etc).

Notably treeherder-client v2.0.1 includes an API URL fix that will
prevent 404s once non-canonical Treeherder API URLs are disabled in
bug 1234233.

jmaher/alert_manager,jmaher/alert_manager,jmaher/alert_manager,jmaher/alert_manager