Add RoomVersions.V3 constant, without enabling it

We add the constant, but don't add it to the known room versions. This
lets us start adding V3 logic, but the servers will never join or create
V3 rooms

synapse/api/constants.py

@@ -104,6 +104,7 @@ class ThirdPartyEntityKind(object):
 class RoomVersions(object):
     V1 = "1"
     V2 = "2"
+    V3 = "3"  # Not currently fully supported, so
     VDH_TEST = "vdh-test-version"
     STATE_V2_TEST = "state-v2-test"