Pin signedjson to <= 1.1.1 as a temporary workaround for #12324

To be reverted after the Synapse 1.56 release.

Signed-off-by: Sean Quah <seanq@element.io>

synapse/python_dependencies.py

@@ -48,7 +48,7 @@ REQUIREMENTS = [
     "unpaddedbase64>=1.1.0",
     "canonicaljson>=1.4.0",
     # we use the type definitions added in signedjson 1.1.
-    "signedjson>=1.1.0",
+    "signedjson>=1.1.0,<=1.1.1",
     "pynacl>=1.2.1",
     "idna>=2.5",
     # validating SSL certs for IP addresses requires service_identity 18.1.