Type defintions for use in refactoring for redaction changes (#6803)

* Bump signedjson to 1.1

... so that we can use the type definitions

* Fix breakage caused by upgrade to signedjson 1.1

Thanks, @illicitonion...

synapse/types.py

@@ -17,7 +17,7 @@ import re
 import string
 import sys
 from collections import namedtuple
-from typing import Dict, Tuple, TypeVar
+from typing import Any, Dict, Tuple, TypeVar
 
 import attr
 from signedjson.key import decode_verify_key_bytes
@@ -43,6 +43,11 @@ T = TypeVar("T")
 StateMap = Dict[Tuple[str, str], T]
 
 
+# the type of a JSON-serialisable dict. This could be made stronger, but it will
+# do for now.
+JsonDict = Dict[str, Any]
+
+
 class Requester(
     namedtuple(
         "Requester", ["user", "access_token_id", "is_guest", "device_id", "app_service"]