Pin to frozendict<2.1.2 (#11625)

... to work around breakage on buster
(https://github.com/Marco-Sulla/python-frozendict/issues/41)

changelog.d/11625.misc

@@ -0,0 +1 @@
+Work around a build problem on Debian Buster.

synapse/python_dependencies.py

@@ -50,7 +50,8 @@ logger = logging.getLogger(__name__)
 REQUIREMENTS = [
     # we use the TYPE_CHECKER.redefine method added in jsonschema 3.0.0
     "jsonschema>=3.0.0",
-    "frozendict>=1",
+    # frozendict 2.1.2 is broken on Debian 10: https://github.com/Marco-Sulla/python-frozendict/issues/41
+    "frozendict>=1,<2.1.2",
     "unpaddedbase64>=1.1.0",
     "canonicaljson>=1.4.0",
     # we use the type definitions added in signedjson 1.1.