Force constraints when installing a package during tox test

The tox deps option grants installation of single dependencies and
requirements, optionally pinned using constraints, before installing
a package, therefore not granting installation of the correct
constraint during the package installation.
To fix that tox 4.4.0 has introduced the constrain_package_deps
option [1]

[1] https://tox.wiki/en/4.12.1/faq.html#using-constraint-files

Change-Id: I770e55db5d6f53174c490749d27830f9209e98cc

tox.ini

@@ -1,8 +1,9 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.4.0
 envlist = py3,functional,pep8
 
 [testenv]
+constrain_package_deps = true
 usedevelop = True
 setenv =
   VIRTUAL_ENV={envdir}
@@ -51,6 +52,7 @@ deps =
     flake8-import-order~=0.18.0 # LGPLv3
     pycodestyle>=2.0.0,<3.0.0 # MIT
     doc8~=1.1.0 # Apache-2.0
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 allowlist_externals = bash
                       {toxinidir}/tools/run_bashate.sh
 commands =