Dropping lower constraints testing

We facing errors related to the new pip resolver, this
topic was discussed on the ML and QA team proposed to
to test lower-constraints [1].

I propose to drop this test because the complexity and recurring pain needed
to maintain that now exceeds the benefits provided by this mechanismes.

Also removing lower constraints and related tox environment as they are
no longer needed.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019390.html

NOTE(hberaud): This backport also includes change
Iefa00bee6f67e9e82f0c703a7aff144c0dcbe1ec squashed  to avoid to
backport multiple patches

Change-Id: Icf48ef72fbfff88dda2714b386dbbfe355dc1acb
(cherry picked from commit 686a8b6182)

.zuul.yaml

@@ -108,7 +108,6 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-victoria-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti

lower-constraints.txt

@@ -1,83 +0,0 @@
-alabaster==0.7.10
-amqp==2.5.2
-appdirs==1.3.0
-Babel==2.3.4
-bandit==1.1.0
-cachetools==2.0.0
-cffi==1.14.0
-cliff==2.8.0
-cmd2==0.8.0
-confluent-kafka==1.3.0
-contextlib2==0.4.0
-coverage==4.0
-debtcollector==1.2.0
-docutils==0.11
-dulwich==0.15.0
-eventlet==0.23.0
-extras==1.0.0
-fasteners==0.7.0
-fixtures==3.0.0
-futurist==1.2.0
-gitdb==0.6.4
-GitPython==1.0.1
-greenlet==0.4.15
-imagesize==0.7.1
-iso8601==0.1.11
-Jinja2==2.10
-keystoneauth1==3.4.0
-kombu==4.6.6
-linecache2==1.0.0
-MarkupSafe==1.1.1
-mock==2.0.0
-mox3==0.20.0
-msgpack-python==0.4.0
-netaddr==0.8.0
-netifaces==0.10.4
-os-client-config==1.28.0
-oslo.concurrency==3.25.0
-oslo.config==5.2.0
-oslo.context==2.19.2
-oslo.i18n==3.15.3
-oslo.log==3.36.0
-oslo.middleware==3.31.0
-oslo.serialization==2.18.0
-oslo.service==1.24.0
-oslo.utils==3.37.0
-oslotest==3.2.0
-Paste==2.0.2
-PasteDeploy==1.5.0
-pbr==2.0.0
-pifpaf==2.2.0
-prettytable==0.7.2
-pycparser==2.18
-Pygments==2.2.0
-pyinotify==0.9.6
-pyngus==2.2.0
-pyparsing==2.1.0
-pyperclip==1.5.27
-python-dateutil==2.5.3
-python-mimeparse==1.6.0
-python-qpid-proton==0.20.0
-python-subunit==1.0.0
-pytz==2013.6
-PyYAML==3.13
-repoze.lru==0.7
-requests==2.14.2
-requestsexceptions==1.2.0
-rfc3986==0.3.1
-Routes==2.3.1
-six==1.10.0
-smmap==0.9.0
-snowballstemmer==1.2.1
-statsd==3.2.1
-stestr==2.0.0
-stevedore==1.20.0
-tenacity==4.4.0
-testscenarios==0.4
-testtools==2.2.0
-traceback2==1.4.0
-unittest2==1.1.0
-vine==1.1.4
-WebOb==1.7.1
-wrapt==1.7.0
-xattr==0.9.2

tox.ini

@@ -118,9 +118,3 @@ deps = -r{toxinidir}/doc/requirements.txt
 [testenv:bindep]
 deps = bindep
 commands = bindep {posargs}
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt