python: add f58plain property to JobID class

Problem: A Python JobID object can't return the f58plain
encoding of a jobid since there's no corresponding class property.

Add an `f58plain` property to the JobID class which returns the
f58plain encoding of the jobid.

Author: garlick

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

@@ -80,6 +80,11 @@ def f58(self):
         """Return RFC19 F58 representation of a JobID"""
         return self.encode("f58")
 
+    @property
+    def f58plain(self):
+        """Return RFC19 F58 representation of a JobID with ASCII prefix"""
+        return self.encode("f58plain")
+
     @property
     def hex(self):
         """Return 0x-prefixed hexadecimal representation of a JobID"""