Get ready for tox 4

Changes:
- made 4.0.0 the tox minversion
- disabling auto discovery by explicitly setting py_modules=[] in
  setup.py to prevent a setuptools issue
  https://github.com/pypa/setuptools/issues/3197#issuecomment-1078770109

This patch makes tox 4 the default so that we can hopefully catch
problems locally before they block the gate.

Change-Id: I8b69991c1377d79a06e2e1617c57180533bb5375

setup.py

@@ -15,4 +15,5 @@ import setuptools
 
 setuptools.setup(
     setup_requires=['pbr>=2.0.0'],
-    pbr=True)
+    pbr=True,
+    py_modules=[])

tox.ini

@@ -1,5 +1,8 @@
 [tox]
-minversion = 3.18.0
+minversion = 4.0.0
+# specify virtualenv here to keep local runs consistent with the
+# gate (it sets the versions of pip, setuptools, and wheel)
+requires = virtualenv>=20.17.1
 envlist = py3,pep8,docs
 ignore_basepython_conflict = true