tox: require tox>=4

Tox 4.0.0 was released on Dec 7, 2022, should be OK to require it.

On the way, since `minversion` is deprecated, replace it with `require`.
https://tox.wiki/en/latest/config.html#min_version

Author: bluetech

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 isolated_build = True
-minversion = 3.20.0
+requires = tox>=4
 distshare = {homedir}/.tox/distshare
 envlist =
     linting