setup: Drop support for python 3.7

setup: Drop support for python 3.7

The main reason to drop this version before it reaches its EOL is so
that we can begin to use unittest.IsolatedAsyncioTestCase to replace
deprecated portions of the tornado.testing module.

A secondary reason is that Python 3.8 introduced support for the
samesite cookie attribute, which allows us to deprecate the
xsrf_token mechanism.