Cleanup py27 support

Make a few cleanups:
- Remove obsolete sections from setup.cfg
- Update requirements, no need for python_version anymore
- Remove install_command from tox.ini, the default is fine.

Change-Id: Iecc1931e93b2254b7b3c95ba1daafd32db204754

requirements.txt

@@ -3,7 +3,7 @@
 # process, which may cause wedges in the gate later.
 
 pbr!=2.1.0  # Apache-2.0
-sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2;python_version>='3.4'  # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,>=1.6.2  # BSD
 graphviz>=0.4,!=0.5.0  # MIT License
 stestr>=2.0.0
 testtools>=0.9.34

setup.cfg

@@ -10,9 +10,3 @@ classifier =
     Intended Audience :: Developers
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
-
-[pbr]
-warnerrors = True
-
-[wheel]
-universal = 1

tox.ini

@@ -8,7 +8,6 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
-install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}