tox: Remove ineffective ignore_basepython_conflict and bump minimum version

Change-Id: I11f52f9c0c50a1470a3fbe74434206b21dc64823
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,14 +1,12 @@
 [tox]
-minversion = 3.1.1
+minversion = 4.6.0
 envlist = py3,pep8,pylint,docs
 skipsdist = True
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 
@@ -38,14 +36,10 @@ norecursedirs  = .tox .venv
 [testenv:venv]
 commands = {posargs}
 
-[testenv:py39]
-basepython = python3.9
-
 [testenv:py312]
 basepython = python3.12
 
 [testenv:cover]
-basepython = python3
 setenv =
     {[testenv]setenv}
     PYTHON=coverage run --source freezerclient --parallel-mode
@@ -57,7 +51,9 @@ commands =
     coverage report
 
 [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 = sphinx-build -b html doc/source doc/build/html
 
 [testenv:pep8]
@@ -79,10 +75,6 @@ enable-extensions=H106,H203
 show-source = True
 exclude = .venv,.tox,dist,doc,*egg,releasenotes
 
-
 [testenv:releasenotes]
-deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-    -r{toxinidir}/requirements.txt
-    -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html