Cleanup py27 support

This commit cleanup below:
- 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
- Add "ignore_basepython_conflict=True" to tox.ini

Change-Id: Ic4fcc1fb15f214ca4204f56ee1ea15dc6a782fc2

setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/watcher/latest/
+python-requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -13,6 +14,8 @@ classifier =
     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
@@ -23,10 +26,6 @@ packages =
 data_files =
     etc/ = etc/*
 
-[global]
-setup-hooks =
-    pbr.hooks.setup_hook
-
 [entry_points]
 oslo.config.opts =
     watcher = watcher.conf.opts:list_opts