Fix the Python 3.5 old-deps build. (#9146)

setuptools 51.0.0 dropped support for Python 3.5.

changelog.d/9146.misc

@@ -0,0 +1 @@
+Fix the Python 3.5 + old dependencies build in CI.

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