tox: Remove ineffective ignore_basepython_conflict and basepython

Change-Id: Ie824a9b04aa4f19d3e1acd4ac03cacc479ebd513
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,10 +1,8 @@
 [tox]
 minversion = 3.18.0
 envlist = py3,pep8
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
@@ -46,13 +44,16 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [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
 setenv = PYTHONHASHSEED=0
 commands =
   rm -rf doc/build
   sphinx-build -E -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html