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: I14a93e67735df0f1b82dd687873ec77d8bfaac78

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}
          PYTHONDONTWRITEBYTECODE = 1
@@ -40,6 +41,7 @@ deps =
     pycodestyle>=2.0.0,<3.0.0 # MIT
     Pygments>=2.2.0 # BSD
     doc8>=0.6.0 # Apache 2.0
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 commands =
     flake8 {posargs}
     doc8 README.rst doc/source --ignore D001
@@ -81,4 +83,4 @@ deps = codespell
 # note(adamcarthur): {posargs} lets us run `tox -ecodespell -- -w` to get codespell
 #             to correct spelling issues in our code it's aware of.
 commands =
-  codespell {posargs}
+  codespell {posargs}