Cleanup Python 2.7 support

OpenStack is dropping the py2.7 support in ussuri cycle.

Make a few cleanups:
- Remove python 2.7 stanza from setup.py
- Add python-requires to setup.cfg so that pypi and pip know
  about support Python version
- Remove ancient sections from setup.cfg
- Remove version_info setting from conf.py, openstackdocstheme does this
  automatically nowadays.

Change-Id: I5b9c159752c932f874015f20822862c70562c2bd

setup.py

@@ -13,17 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# THIS FILE IS MANAGED BY THE GLOBAL REQUIREMENTS REPO - DO NOT EDIT
 import setuptools
 
-# In python < 2.7.4, a lazy loading of package `pbr` will break
-# setuptools if some other modules registered functions in `atexit`.
-# solution from: http://bugs.python.org/issue15881#msg170215
-try:
-    import multiprocessing  # noqa
-except ImportError:
-    pass
-
 setuptools.setup(
     setup_requires=['pbr>=2.0.0'],
     pbr=True)