[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: Ia4ce42a20417d69efd50ef563aabd8cffbb7b346

doc/requirements.txt

@@ -4,5 +4,5 @@
 
 openstackdocstheme>=1.19.0 # Apache-2.0
 reno!=2.3.1,>=1.8.0 # 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 = 2.0
+minversion = 3.1
 envlist = py3,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3