Canonicaljson dependency fix

[mcalinghee]

Fix from : matrix-org#13172

When installing synapse it will bring canonicaljson==2.0.0 and raise the following issue

Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:     _check_size_limits(event)
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:   File "/opt/synapse/env/lib/python3.7/site-packages/synapse/event_auth.py", line 276, in _check_size_limits
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:     if len(encode_canonical_json(event.get_pdu_json())) > MAX_PDU_SIZE:
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:   File "/opt/synapse/env/lib/python3.7/site-packages/canonicaljson/__init__.py", line 84, in encode_canonical_json
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:     s = _canonical_encoder.encode(data)
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:   File "/usr/lib/python3.7/json/encoder.py", line 199, in encode
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:     chunks = self.iterencode(o, _one_shot=True)
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:   File "/usr/lib/python3.7/json/encoder.py", line 257, in iterencode
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:     return _iterencode(o, 0)
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:   File "/opt/synapse/env/lib/python3.7/functools.py", line 827, in wrapper
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:     return dispatch(args[0].__class__)(*args, **kw)
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:   File "/opt/synapse/env/lib/python3.7/site-packages/canonicaljson/__init__.py", line 31, in _preprocess_for_serialisation
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]:     "Object of type %s is not JSON serializable" % obj.__class__.__name__
Mar 15 17:55:41 tchap-agent1 matrix-synapse_event_creator[4864]: TypeError: Object of type frozendict is not JSON serializable

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