Enable parallel docs builds and bump reno min version (#4477)

* Enable parallel docs builds and bump reno min version

In the recent reno release 3.1.0 the sphinx extension config option
'parallel_read_safe' was set to true. [1] This makes it possible to run
sphinx builds with reno in parallel without a warning being raised. This
commit does just that and adds '-j auto' to the default sphinx build
command in the tox docs job. This should hopefully speed up the docs
builds in CI (although the degree to which is limited by the available
resources in the CI nodes) and for local builds.

[1] https://opendev.org/openstack/reno/commit/cc09e3d38c2c1046a529270f133de525c0692809

* Don't install reno with 3.5

Reno 3.0.0 dropped support for python 3.5 and setting a minimum version
above that is causing the python 3.5 jobs to fail. Reno is only used for
docs jobs which run with python 3.7 and we don't need to install it as
part of a normal test run. This commit makes the reno test requirement
only installed on python > 3.5.

requirements-dev.txt

@@ -16,7 +16,7 @@ cython>=0.27.1
 pylatexenc>=1.4
 ddt>=1.2.0,!=1.4.0
 seaborn>=0.9.0
-reno>=2.11.0
+reno>=3.1.0;python_version>'3.5'
 Sphinx>=1.8.3
 sphinx-rtd-theme>=0.4.0
 sphinx-tabs>=1.1.11

tox.ini

@@ -44,4 +44,4 @@ deps =
   qiskit-aer
   qiskit-ibmq-provider
 commands =
-  sphinx-build -W -b html docs/ docs/_build/html {posargs}
+  sphinx-build -W -b html -j auto docs/ docs/_build/html {posargs}