drop lower-constraints

The new pip resolver is extremely slow at calculating the
lower-constraints. Some other OpenStack projects have already dropped
the job. Let's do the same for reno.

Signed-off-by: Doug Hellmann <doug@doughellmann.com>
Change-Id: I489cbdf603991888c13b7730cfeb590fea175c25

.zuul.yaml

@@ -3,4 +3,3 @@
       - openstack-python3-victoria-jobs
       - openstack-cover-jobs
       - publish-openstack-docs-pti
-      - openstack-lower-constraints-jobs

lower-constraints.txt

@@ -1,5 +0,0 @@
-docutils==0.11
-dulwich==0.15.0
-PyYAML==5.3.1
-Sphinx==2.0.0
-stestr==2.1.0

tox.ini

@@ -21,11 +21,6 @@ commands =
   flake8
   reno -q lint
 
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  {[testenv]deps}
-
 [testenv:venv]
 commands = {posargs}
 
@@ -40,12 +35,6 @@ commands =
    coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-# NOTE(dhellmann): Build our own documentation using the
-# lower-constraints list as a hacky way to test the sphinx extension
-# module, since we don't have separate unit tests for it.
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  {[testenv]deps}
 commands = sphinx-build -a -W -E -b html doc/source doc/build/html
 
 [testenv:debug]