tox: Use new constraints option

It is a preferred way to define the constraints file.

Change-Id: Id51d137847bfb046cf54914d0d7c3ff8809dadc8
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -1,12 +1,13 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.28.0
 envlist = cover,docs,pep8,py3,pylint,update-states
 
 [testenv]
 # We need to install a bit more than just `test' because those drivers have
 # custom tests that we always run
+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}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 commands =
@@ -54,7 +55,6 @@ commands =
 
 [testenv:releasenotes]
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
@@ -62,9 +62,9 @@ commands =
 [testenv:pep8]
 skip_install = true
 deps =
-    pre-commit
+  pre-commit
 commands =
-    pre-commit run -a
+  pre-commit run -a
 
 [testenv:pylint]
 deps =