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: I3a9e4748c1cd7aad4c88809b9d3652471b7a3e0d

tox.ini

@@ -1,10 +1,11 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.4.0
 envlist = py3,pep8
 ignore_basepython_conflict=true
 
 [testenv]
 basepython = python3
+constrain_package_deps = true
 usedevelop = True
 setenv =
    VIRTUAL_ENV={envdir}
@@ -20,6 +21,7 @@ deps=
     hacking~=6.0.0 # Apache-2.0
     flake8-import-order>=0.17.1 # LGPLv3
     pycodestyle>=2.0.0,<3.0.0 # MIT
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 commands = flake8 {posargs}
 
 [testenv:venv]