nova-specs should use upper-constraints

A recent release of python-subunit 1.4.3 caused
stestr to break due to an undeclared transitive dep.
this is fixed in stestr 4.1.0 but it highlighted that
nova-specs is not using uc when installing and as a result
is prone to failures that wont happen in other tox venvs.

This change simply enables the use of upper-constraints
to prevent future breakages like this.

Change-Id: Ic8a19b8426d3d91f82e907ace62f846a9d6c7282

tox.ini

@@ -7,7 +7,9 @@ skipsdist = True
 basepython = python3
 #usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 allowlist_externals =
   find
   bash
@@ -25,7 +27,7 @@ commands = bash tools/fast-specs.sh
 
 [testenv:pep8]
 deps =
-  -r{toxinidir}/doc/requirements.txt
+  {[testenv]deps}
   -r{toxinidir}/test-requirements.txt
 commands =
   find . -type f -name "*.pyc" -delete