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: I98e55372488fb0acc24d08b23ca6b41f7cb2764a

tox.ini

@@ -1,9 +1,10 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.4.0
 envlist = py3,pep8
 ignore_basepython_conflict=true
 
 [testenv]
+constrain_package_deps = true
 usedevelop = True
 basepython = python3
 setenv =
@@ -21,6 +22,7 @@ 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]
@@ -73,4 +75,4 @@ deps = codespell
 # note(JayF): {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}