Add workaround for towncrier

Towncrier is trying to access `importlib_resources.is_resource`,
but this was removed in the latest release.

Twisted maintainers already tackled this problem,
but we are waiting for a new release.
See twisted/towncrier#528.

tox.ini

@@ -40,6 +40,8 @@ extras =
 	docs
 	testing
 changedir = docs
+deps =
+	importlib_resources < 6  # twisted/towncrier#528 (waiting for release)
 commands =
 	python -m sphinx -W --keep-going . {toxinidir}/build/html
 	python -m sphinxlint
@@ -50,6 +52,7 @@ deps =
 	towncrier
 	bump2version
 	jaraco.develop >= 7.23
+	importlib_resources < 6  # twisted/towncrier#528 (waiting for release)
 passenv = *
 commands =
 	python tools/finalize.py