python: allow JobID to take a JobID

Problem: The JobID class allows users to pass in a jobid in many
forms.  However, it does not allow JobID to be initialized via another
JobID object, in the event that is what the user passed in.

Allow JobID to take a JobID for initialization.

Author: chu11

src/bindings/python/flux/job/JobID.py

@@ -53,6 +53,8 @@ class JobID(int):
     """
 
     def __new__(cls, value):
+        if isinstance(value, JobID):
+            return cls(value.orig_str)
         if isinstance(value, int):
             jobid = value
         else: