tox: Remove ineffective ignore_basepython_conflict and bump minimum version

Change-Id: Ic391e0a716d560d8fb1c71578e968bedb1c9ac52
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,linters,functional
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = False
 commands =
     /usr/bin/find . -type f -name "*.pyc" -delete
@@ -51,9 +49,7 @@ commands =
 extensions = .rst
 
 [testenv:releasenotes]
-deps =
-    -r{toxinidir}/doc/requirements.txt
-    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+deps = {[testenv:docs]deps}
 commands =
     sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html