tox: Remove ineffective ignore_basepython_conflict

Change-Id: I538e63c8d8c7e8b8f31dbf10aea5947bfdd64cb3
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,11 +1,9 @@
 [tox]
-minversion = 4.0
+minversion = 4.6.0
 envlist = docs,pep8,py3
 skipsdist = True
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
@@ -20,7 +18,9 @@ commands =
 commands = {posargs}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps=
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/build/html doc/build/doctrees
   find . -type f -name "*.pyc" -delete
@@ -28,7 +28,7 @@ commands =
 
 [testenv:pep8]
 skip_install = true
-deps = -r{toxinidir}/test-requirements.txt
+deps = {[testenv:docs]deps}
 commands =
   flake8 {posargs}