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

Change-Id: I57dc2210d369cb89ad2de7d2c304a8f225fd6adf
diff --git a/setup.cfg b/setup.cfg
index bf0b008..7d2ca94 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -6,6 +6,7 @@
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/designate-tempest-plugin/latest/
+python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Environment :: No Input/Output (Daemon)
@@ -14,15 +15,13 @@
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     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
     Topic :: Internet :: Name Service (DNS)
 
-[global]
-setup-hooks =
-    pbr.hooks.setup_hook
-
 [files]
 packages =
     designate_tempest_plugin
@@ -31,15 +30,7 @@
 tempest.test_plugins =
     designate = designate_tempest_plugin.plugin:DesignateTempestPlugin
 
-[build_sphinx]
-all_files = 1
-build-dir = doc/build
-source-dir = doc/source
-
 [egg_info]
 tag_build =
 tag_date = 0
 tag_svn_revision = 0
-
-[wheel]
-universal = 1