Require tox version 4 for consistency

Even if our `tox.ini` is compatible with tox 3, the command line
format changed with tox 4. For consistency in layout, usage and
documentation, it seems best to update requirements to version 4.

See: https://tox.wiki/en/latest/upgrading.html#updating-usage-with-e

tox.ini

 [tox]
+requires =
+  tox>4
 envlist=flake8,black
 
 [testenv]
 basepython = python3
-passenv = SPHINXOPTS,SPHINXOPTCOLOR,JAVA_HOME
+passenv =
+  SPHINXOPTS
+  SPHINXOPTCOLOR
+  JAVA_HOME
 
 [testenv:sphinx]
 extras =