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

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

Watcher is ready with python 3 and ok to drop the
python 2.7 support.

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

Depends-On: https://review.opendev.org/#/c/693631/

Change-Id: I603c6d2c22779e8ef2e70eb6369fc521a77c9c3a

doc/requirements.txt

@@ -2,7 +2,6 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 openstackdocstheme>=1.20.0 # Apache-2.0
-sphinx>=1.8.0,<2.0.0;python_version=='2.7' # BSD
 sphinx>=1.8.0,!=2.1.0;python_version>='3.4' # BSD
 sphinxcontrib-pecanwsme>=0.8.0 # Apache-2.0
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD