Use the master constraint again in tox.ini

WHile releasing the new tag for Tempest, we modified the
constraint in tox to be used of stable/ussuri
- https://review.opendev.org/#/c/720578/

Now new tag is released and we can bring back the master
constraint in tox.ini so that Tempest master us master constraint.

Change-Id: Ie17d4830cac1902371123c8acb4d3106ba2fd300
diff --git a/tox.ini b/tox.ini
index e324c91..e861c84 100644
--- a/tox.ini
+++ b/tox.ini
@@ -11,7 +11,7 @@
     VIRTUAL_ENV={envdir}
     OS_TEST_PATH=./tempest/test_discover
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
 
 [testenv]
@@ -28,7 +28,7 @@
 install_command = pip install {opts} {packages}
 whitelist_externals = *
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 commands =
@@ -263,7 +263,7 @@
 
 [testenv:venv]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
@@ -278,7 +278,7 @@
 
 [testenv:docs]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
@@ -348,7 +348,7 @@
 
 [testenv:releasenotes]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =