tox: Remove ineffective ignore_basepython_conflict and bump minimum version

Change-Id: Iaf79935ad91c0a4ec6f8d6c16a2fc7dbfedefe8a
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,11 +1,9 @@
 [tox]
-minversion = 4.0
+minversion = 4.6.0
 skipsdist = True
 envlist = docs,molecule
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = False
 install_command =
     pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
@@ -33,7 +31,9 @@ setenv =
     WORKING_DIR={toxinidir}
 
 [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=
     bash -c "rm -rf doc/build"
     doc8 doc
@@ -44,7 +44,7 @@ commands=
 extensions = .rst
 
 [testenv:releasenotes]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html