Drop lower-constraints

Lower-constraints is not a requirement of the OpenStack Python PTI
[0] and there currently is a discussion on the mailing list [1]
about dropping the test, with the oslo team already having done
so [2].

The new dependency resolver in pip fails due to incompatible
dependency versions in our lower-constraints file, meaning that
we were never providing any real guarantees with it.

To unblock the CI, I am disabling lower-constraints job for now,
with the option to reenable it in case we fix the constraints,
and based on the outcome of the mailing list discussions and
consensus.

[0]. https://governance.openstack.org/tc/reference/pti/python.html
[1]. http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019672.html
[2]. http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019659.html

Change-Id: I588fa809839cf3112dae24e356547100f7e89bc5

tox.ini

@@ -26,7 +26,7 @@ passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 commands =
     doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
     flake8
-    bandit -r watcher -x watcher/tests/* -n5 -ll -s B320,B322
+    bandit -r watcher -x watcher/tests/* -n5 -ll -s B320
 
 [testenv:venv]
 setenv = PYTHONHASHSEED=0
@@ -132,9 +132,3 @@ commands = sphinx-build -a -W -E -d releasenotes/build/doctrees --keep-going -b
 [testenv:bandit]
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r watcher -x watcher/tests/* -n5 -ll -s B320
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt