Stop using openstack upper constraints

We want to be able to support older Python versions than those that are
possible with the current OpenStack constraints. Run unit tests without
constraints instead.

Change-Id: I4237a56ff9733b5e3c9521193cc5e9758c28095e

tox.ini

@@ -8,7 +8,6 @@ ignore_basepython_conflict = True
 usedevelop = True
 basepython = python3
 deps=
-      -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
       -r{toxinidir}/requirements.txt
       -r{toxinidir}/test-requirements.txt
 commands= stestr run --slowest {posargs}
@@ -73,9 +72,3 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
-
-# Run without upper-constraints on python3.5, as since Victoria that
-# now is python3.6 only
-[testenv:py35]
-basepython = python3.5
-deps = {[testenv:lower-constraints]deps}