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
  * Some other sections are obsolete
- Update classifiers
- Update requirements, no need for python_version anymore
- Remove html_last_updated_fmt from conf.py, this is not needed with
  openstackdocstheme anymore; update openstackdocstheme requirement

Change-Id: Ib3ea5712db7075a67a4f9da74a6788822658e97a

setup.cfg

@@ -7,8 +7,11 @@ license = Apache License, Version 2.0
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/python-freezerclient/latest/
+python-requires = >=3.6
 classifier =
     Programming Language :: Python
+    Programming Language :: Python :: Implementation :: CPython
+    Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7

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)

test-requirements.txt

@@ -5,10 +5,7 @@ flake8<2.6.0,>=2.5.4 # MIT
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 mock>=2.0.0 # BSD
-#pylint==1.9.2 # GPLv2
 stestr>=2.0.0 # Apache-2.0
 testtools>=2.2.0 # MIT
-astroid==1.6.5;python_version<"3.0" # LGPLv2.1
-astroid==2.1.0;python_version>="3.0" # LGPLv2.1
-pylint==1.9.2;python_version<"3.0" # GPLv2
-pylint==2.2.0;python_version>="3.0" # GPLv2
+astroid==2.1.0 # LGPLv2.1
+pylint==2.2.0 # GPLv2