[ussuri][goal] Drop python 2.7 support and testing

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

mistral-tempest-plugin is ready with python 3 and ok to drop the
python 2.7 support.

Removing the USE_PYTHON3=True from base job so that it can
keep running  on py3 for < ussuri branches.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I060587c96baba096525211ffccbe29f71b38760d

.zuul.yaml

@@ -21,7 +21,6 @@
         MYSQL_ROOT_PW: secretdatabase
         MISTRAL_USE_MOD_WSGI: true
         MISTRAL_RPC_IMPLEMENTATION: oslo
-        USE_PYTHON3: true
       devstack_services:
         heat: true
         h-api: true

doc/requirements.txt

@@ -1,4 +1,4 @@
-sphinx>=1.8.0,!=2.1.0;python_version>='3.4' # BSD
+sphinx>=1.8.0,!=2.1.0 # BSD
 sphinxcontrib-httpdomain>=1.3.0 # BSD
 sphinxcontrib-pecanwsme>=0.8.0 # Apache-2.0
 openstackdocstheme>=1.30.0 # Apache-2.0

releasenotes/notes/drop-py-2-7-324d8df63fe680c0.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of mistral-tempest-plugin
+    to support py2.7 is OpenStack Train. The minimum version of Python now
+    supported by mistral-tempest-plugin is Python 3.5.

setup.cfg

@@ -14,7 +14,6 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6