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: I578946bc780acfcc63388201a35e7c6fa0e10a35

tox.ini

@@ -1,8 +1,9 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.4.0
 envlist = py3,pep8
 
 [testenv]
+constrain_package_deps = true
 setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
          LANGUAGE=en_US
@@ -29,6 +30,7 @@ deps =
     flake8-import-order>=0.17.1 # LGPLv3
     pycodestyle>=2.0.0,<3.0.0 # MIT
     Pygments>=2.2.0 # BSD
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 commands =
     flake8 {posargs}
     doc8 doc/source CONTRIBUTING.rst README.rst