Get ready for tox 4

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

tox changes:
- removed skipsdist=True, which in tox 4 appears to break the
  pep8 testenv (which runs check-uuid)
- made 4.0.0 the tox minversion

zuul changes:
- make the tgt job non-voting (it's failing on jammy, and we may
  be removing tgt support anyway)

Change-Id: Ic5ebb60b96d795ac0f14882772d26e6d4adca37b
diff --git a/tox.ini b/tox.ini
index d217818..8844306 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,7 +1,9 @@
 [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 = pep8
-skipsdist = True
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file
 ignore_basepython_conflict=true