Try to test against my signedjson PR

Author: David Robertson

synapse/python_dependencies.py

@@ -48,7 +48,7 @@
     "unpaddedbase64>=1.1.0",
     "canonicaljson>=1.4.0",
     # we use the type definitions added in signedjson 1.1.
-    "signedjson>=1.1.0",
+    "signedjson=https://github.com/matrix-org/python-signedjson/archive/refs/heads/dmr/typing3.zip",
     "pynacl>=1.2.1",
     "idna>=2.5",
     # validating SSL certs for IP addresses requires service_identity 18.1.