Update zope-testing requirement from <5,>=4 to >=4,<6

Updates the requirements on [zope-testing](https://github.com/zopefoundation/zope.testing) to permit the latest version.
- [Release notes](https://github.com/zopefoundation/zope.testing/releases)
- [Changelog](https://github.com/zopefoundation/zope.testing/blob/master/CHANGES.rst)
- [Commits](zopefoundation/zope.testing@4.0.0...5.0.1)

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

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

Author: dependabot[bot]

setup.py

@@ -64,7 +64,7 @@ def read(path):
                     'geojson>=2.5.0,<4',
                     'backports.zoneinfo<1; python_version<"3.9"'],
         test=['tox>=3,<5',
-              'zope.testing>=4,<5',
+              'zope.testing>=4,<6',
               'zope.testrunner>=5,<6',
               'zc.customdoctests>=1.0.1,<2',
               'createcoverage>=1,<2',