Pin to twisted 17.1 or later

Fixes https://github.com/matrix-org/synapse/issues/3741.

synapse/python_dependencies.py

@@ -39,7 +39,7 @@ REQUIREMENTS = {
     "signedjson>=1.0.0": ["signedjson>=1.0.0"],
     "pynacl>=1.2.1": ["nacl>=1.2.1", "nacl.bindings"],
     "service_identity>=1.0.0": ["service_identity>=1.0.0"],
-    "Twisted>=16.0.0": ["twisted>=16.0.0"],
+    "Twisted>=17.1.0": ["twisted>=17.1.0"],
 
     # We use crypto.get_elliptic_curve which is only supported in >=0.15
     "pyopenssl>=0.15": ["OpenSSL>=0.15"],