Switch to newer openstackdocstheme and reno versions

Switch to openstackdocstheme 2.2.0 and reno 3.1.0 versions. Using
these versions will allow especially:
* Linking from HTML to PDF document
* Allow parallel building of documents
* Fix some rendering problems

Update Sphinx version as well.

Remove docs requirements from lower-constraints, they are not needed
during install or test but only for docs building.

openstackdocstheme renames some variables, so follow the renames
before the next release removes them. A couple of variables are also
not needed anymore, remove them.

Set openstackdocs_pdf_link to link to PDF file. Note that
the link to the published document only works on docs.openstack.org
where the PDF file is placed in the top-level html directory. The
site-preview places the PDF in a pdf directory.

Set openstackdocs_auto_name to use 'project' as name.

Change-Id: I80932c070dbddf9a75f64b0a4d4c614efd5a06ff
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 12c20ec..ff8c025 100755
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -85,9 +85,10 @@
 html_theme = 'openstackdocs'
 
 # openstackdocstheme options
-repository_name = 'openstack/patrole'
-bug_project = 'patrole'
-bug_tag = ''
+openstackdocs_repo_name = 'openstack/patrole'
+openstackdocs_pdf_link = True
+openstackdocs_bug_project = 'patrole'
+openstackdocs_bug_tag = ''
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'patroledoc'