Bump protocol version.

Author: filmor

docs/messaging.rst

@@ -21,7 +21,7 @@ Versioning
 
 The Jupyter message specification is versioned independently of the packages
 that use it.
-The current version of the specification is 5.2.
+The current version of the specification is 5.3.
 
 .. note::
    *New in* and *Changed in* messages in this document refer to versions of the

jupyter_client/_version.py

@@ -1,5 +1,5 @@
 version_info = (5, 1, 0)
 __version__ = '.'.join(map(str, version_info))
 
-protocol_version_info = (5, 2)
+protocol_version_info = (5, 3)
 protocol_version = "%i.%i" % protocol_version_info