diff --git a/changelog.d/9146.misc b/changelog.d/9146.misc
new file mode 100644
index 0000000000..7af29baa30
--- /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 297136fcc5..5210e7b860 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