Use upper-constraints in tox runs

This fix stolen from:
https://review.opendev.org/#/c/658567/

The upper-constraints file wasn't being enforced
when installing dependencies in tox runs which can
lead to failures like installing Sphinx>2.0 in py27
where it's not supported.

This is loosely based on novaclient change
I8be883215f27abb58d15b85e8542cbdf32000bac for the
same kind of issue.

Change-Id: I47d12348e5a80048785a93842f0dfd3060bdbc24

tox.ini

@@ -5,9 +5,14 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+passenv = ZUUL_CACHE_DIR
+          REQUIREMENTS_PIP_LOCATION
+install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/test-requirements.txt
+deps = 
+       -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+       -r{toxinidir}/test-requirements.txt
+       -r{toxinidir}/requirements.txt
 
 [testenv:venv]
 commands = {posargs}