Chore(deps-dev): Update zope-testing requirement from <6,>=4 to >=4,<7

Updates the requirements on [zope-testing](https://github.com/zopefoundation/zope.testing) to permit the latest version.
- [Changelog](https://github.com/zopefoundation/zope.testing/blob/master/CHANGES.rst)
- [Commits](zopefoundation/zope.testing@4.0.0...6.0)

---
updated-dependencies:
- dependency-name: zope-testing
  dependency-version: '6.0'
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

setup.py

@@ -73,7 +73,7 @@ def read(path):
             "stopit>=1.1.2,<2",
             "pytz",
             "zc.customdoctests>=1.0.1,<2",
-            "zope.testing>=4,<6",
+            "zope.testing>=4,<7",
             "zope.testrunner>=5,<8",
         ],
     },