diff --git a/changelog.d/9146.misc b/changelog.d/9146.misc
new file mode 100644
index 0000000000000000000000000000000000000000..7af29baa30e80718392ebd8aebae96d43177582e
--- /dev/null
+++ b/changelog.d/9146.misc
@@ -0,0 +1 @@
+Fix the Python 3.5 + old dependencies build in CI.
diff --git a/tox.ini b/tox.ini
index 297136fcc5515ca42135a8b25a40350a2e76c141..5210e7b86016ee5b4af0c39319fbb01a6e40a323 100644
--- a/tox.ini
+++ b/tox.ini
@@ -103,6 +103,9 @@ usedevelop=true
 [testenv:py35-old]
 skip_install=True
 deps =
+    # Ensure a version of setuptools that supports Python 3.5 is installed.
+    setuptools < 51.0.0
+
     # Old automat version for Twisted
     Automat == 0.3.0