Switch to newer openstackdocstheme and reno versions

Switch to openstackdocstheme 2.2.1 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.

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 False to use 'project' variable as name.

Change pygments_style to 'native' since old theme version always used
'native' and the theme now respects the setting and using 'sphinx' can
lead to some strange rendering.

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.

See also
http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014971.html

Change-Id: Ia9a3fb804fb59bb70edc150a3eb20c07a279170b

api-ref/source/conf.py

@@ -22,9 +22,6 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 
-from watcher import version as watcher_version
-
-
 extensions = [
     'openstackdocstheme',
     'os_api_ref',
@@ -46,21 +43,13 @@ project = u'Infrastructure Optimization API Reference'
 copyright = u'2010-present, OpenStack Foundation'
 
 # openstackdocstheme options
-repository_name = 'openstack/watcher'
-bug_project = 'watcher'
-bug_tag = ''
-
-# The version info for the project you're documenting, acts as replacement for
-# |version| and |release|, also used in various other places throughout the
-# built documents.
-#
-# The full version, including alpha/beta/rc tags.
-release = watcher_version.version_info.release_string()
-# The short X.Y version.
-version = watcher_version.version_string
+openstackdocs_repo_name = 'openstack/watcher'
+openstackdocs_auto_name = False
+openstackdocs_bug_project = 'watcher'
+openstackdocs_bug_tag = ''
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+pygments_style = 'native'
 
 # -- Options for HTML output --------------------------------------------------
 
@@ -75,10 +64,6 @@ html_theme_options = {
     "sidebar_mode": "toc",
 }
 
-# If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
-# using the given strftime format.
-html_last_updated_fmt = '%Y-%m-%d %H:%M'
-
 # -- Options for LaTeX output -------------------------------------------------
 
 # Grouping the document tree into LaTeX files. List of tuples