Falcon 2.0.0 introduces some breaking changes. The relevant ones here are:
- falcon.testing.TestCase.api property was removed
- falcon.testing.TestBase class was removed
This commit also switches the project to use stestr, update sphinx dependency
and add lower-constraints job
Change-Id: I34255a603c069ed3b1e52017634ff01147473221
Story: 2005695
Task: 35688