tox: Remove ineffective ignore_basepython_conflict and bump minimum version

Change-Id: Ie63a23999801c7e8be3de1183b097904ab5adfb8
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,11 +1,9 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.6.0
 skipsdist = True
 envlist = docs
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 deps =
     -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
@@ -28,7 +26,9 @@ setenv =
     VIRTUAL_ENV={envdir}
 
 [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