tox: Use new constraints option

It is a preferred way to define the constraints file.

Change-Id: I98f1df943a43c96197f7d2769fb9fa777ad5892b
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -1,10 +1,11 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.28.0
 envlist = py3,pypy,pep8
 
 [testenv]
+constraints =
+  {env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
@@ -44,7 +45,6 @@ commands =
 allowlist_externals =
   rm
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf releasenotes/build