Cleanup py27 support

This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove python 2.7 stanza from setup.py
- Remove obsolete sections from setup.cfg
- Update classifiers
- Use newer openstackdocstheme and Sphinx versions
- Cleanup */source/conf.py to remove now obsolete content.
- Remove install_command from tox.ini, the default is fine
- Remove hacking requirements from lower-constraints, they
  are not needed for install

Change-Id: I150a5ee2cd08abf5ce9cf9daf2835007dea0dffd
diff --git a/doc/requirements.txt b/doc/requirements.txt
index ae644f5..d155165 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,8 +1,8 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2 # BSD
-openstackdocstheme>=1.20.0 # Apache-2.0
+sphinx>=2.0.0,!=2.1.0 # BSD
+openstackdocstheme>=2.0.0 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
 sphinxcontrib-apidoc>=0.2.0  # BSD
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD