Don't use upper-constraints when installing package

We only want to use upper-constraints when installing dependencies, not
the os-resource-classes package itself, otherwise we get:

   Could not satisfy constraints for 'os-resource-classes':
   installation from path or url cannot be constrained to a version

Change-Id: I5360e041b6b81c5686e2700aa89de8514a69dd9a
Closes-Bug: #1809401

tox.ini

@@ -5,14 +5,16 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
    OS_STDOUT_CAPTURE=1
    OS_STDERR_CAPTURE=1
    OS_TEST_TIMEOUT=60
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
 [testenv:pep8]