Cleanup py27 support

Make a few cleanups:
- Remove python 2.7 stanza from setup.py
- Add requires on python >= 3.6 to setup.cfg so that pypi and pip
  know about the requirement
- Remove obsolete sections from setup.cfg:
  * Wheel is not needed for python 3 only repo

Change-Id: Ie5bf4b5226a06d828ec24a6134d7ea8a3f45c8db

setup.cfg

@@ -3,6 +3,7 @@ name = sahara-image-elements
 summary = Image elements for Sahara
 description-file = README.rst
 license = Apache Software License
+python-requires = >=3.6
 classifiers =
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators
@@ -19,6 +20,3 @@ scripts =
 
 data_files =
     share/sahara-elements/elements = elements/*
-
-[wheel]
-universal = 1

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)