Skip blazar install in docs and pep8 jobs

After installing doc build requirements with upper constraints, tox was
installing blazar without constraints, which was pulling Jinja2 3.1.0
(released today) as a Flask dependency. This caused a docs build failure
on stable/wallaby, because Jinja2 dropped support for environmentfilter
used by the version of Sphinx in upper constraints.

Use skip_install to instruct tox not to install blazar when not needed.
This requires adding oslo.config and oslo.policy to doc/requirements.txt
because we use their sphinx extensions.

Also make lower-constraints job non-voting in the gate.

Change-Id: Ic55dba0038a2209e9fb291a604a5c1da57607e1e
(cherry picked from commit 2c3d94ce3b)

.zuul.yaml

@@ -19,3 +19,5 @@
       jobs:
         - blazar-tempest-plugin-base
         - blazar-tempest-plugin-ipv6-only
+        - openstack-tox-lower-constraints:
+            voting: false

doc/requirements.txt

@@ -3,3 +3,5 @@ reno>=3.1.0 # Apache-2.0
 sphinx>=2.0.0,!=2.1.0 # BSD
 openstackdocstheme>=2.2.1 # Apache-2.0
 os-api-ref>=1.4.0 # Apache-2.0
+oslo.config>=6.8.0 # Apache-2.0
+oslo.policy>=3.6.0 # Apache-2.0

tox.ini

@@ -33,12 +33,14 @@ commands =
   coverage report
 
 [testenv:pep8]
+skip_install = True
 commands = flake8 {posargs}
 
 [testenv:venv]
 commands = {posargs}
 
 [testenv:docs]
+skip_install = True
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
        -r{toxinidir}/doc/requirements.txt
 commands =
@@ -47,6 +49,7 @@ commands =
     sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
+skip_install = True
 deps = {[testenv:docs]deps}
 allowlist_externals = make
 commands =