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/lower-constraints.txt b/lower-constraints.txt
index 2b77dff..86d7dbd 100644
--- a/lower-constraints.txt
+++ b/lower-constraints.txt
@@ -16,9 +16,7 @@
 extras==1.0.0
 fasteners==0.14.1
 fixtures==3.0.0
-flake8==2.6.2
 future==0.16.0
-hacking==1.0.0
 idna==2.6
 imagesize==1.0.0
 iso8601==0.1.12
@@ -36,7 +34,7 @@
 netifaces==0.10.6
 nose==1.3.7
 nosexcover==1.0.10
-openstackdocstheme==1.20.0
+openstackdocstheme==2.0.0
 os-client-config==1.29.0
 oslo.concurrency==3.26.0
 oslo.config==5.2.0
@@ -49,11 +47,9 @@
 oslotest==3.2.0
 paramiko==2.4.1
 pbr==2.0.0
-pep8==1.5.7
 prettytable==0.7.2
 pyasn1==0.4.2
 pycparser==2.18
-pyflakes==0.8.1
 Pygments==2.2.0
 pyinotify==0.9.6
 PyNaCl==1.2.1
@@ -70,7 +66,7 @@
 rfc3986==1.1.0
 six==1.11.0
 snowballstemmer==1.2.1
-Sphinx==1.6.5
+Sphinx==2.0.0
 sphinxcontrib-websupport==1.0.1
 stestr==2.0.0
 stevedore==1.20.0