tox.ini: use minversion instead of requires

This allows using the "system" tox, if it's recent enough, instead of
always provisioning an internal .tox venv with tox 4.

tox.ini

 [tox]
-requires =
-  tox>4
+minversion=4
 envlist=black,flake8,mypy,py3-{minimal,full-cover}
 
 [testenv]