[ussuri][goal] More cleanup for python 2.7 drop

Cleanup requirement and set ignore_basepython_conflict = True
in tox to avoid python version conflict warning which going
to be error in future.

Change-Id: Iae7e0bb9e8af52ec3aafba7508305d8692d3c762

doc/requirements.txt

@@ -7,5 +7,5 @@
 # be installed in a specific order.
 openstackdocstheme>=1.19.0 # Apache-2.0
 reno>=2.5.0 # Apache-2.0
-sphinx>=1.8.0,!=2.1.0;python_version>='3.4'  # BSD
+sphinx>=1.8.0,!=2.1.0 # BSD
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD

tox.ini

 [tox]
-minversion = 2.5.0
+minversion = 3.1
 skipsdist = True
 envlist = py37,pep8,pypy
+ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3