Unpin tox

tox 4.x seems less likely to break things than 3.x these days.

Change-Id: Ice7365a341dd08dff17c2cae918e86b084d3cc45
Signed-off-by: Tim Burke <tim.burke@gmail.com>

Author: tipabu

.zuul.yaml

@@ -10,9 +10,6 @@
     # for installation.
     required-projects:
       - opendev.org/openstack/python-swiftclient
-    vars:
-      # New tox keeps breaking things as of 2023-01
-      ensure_tox_version: '<4'
 
 - job:
     name: swiftclient-functional