CI: Fix py36 and py37 jobs

The openstack/requirements repo has dropped support for these versions
of python; as a result, some constraints have moved forward to versions
requiring py38+.

Fortunately, the Python ecosystem has gotten much better about specifying
Python version requirements, so we should be clear to just trust the
package metadata to get us a useful version.

Change-Id: I44941d9f7cb145631a0daae2d12cfe85be854f6f

tox.ini

@@ -24,6 +24,12 @@ passenv = SWIFT_*
 commands =
     python -m flake8 swiftclient test
 
+[testenv:{py36,py37}]
+# Drop the use of constraints; most dependencies have dropped support for
+# these versions already, and have updated their metadata to reflect that
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+
 [testenv:venv]
 commands = {posargs}