python: support raw description formatter with help_formatter()

Problem: There is currently no way to get raw description and
epilog formatting with the FluxHelpFormatter class returned from
help_formatter().

Add a FluxRawDescriptionHelpFormatter class that inherits from both
FluxHelpFormatter and RawDescriptionHelpFormatter for this purpose.
Return this class instead of FluxHelpFormatter if a new optional
raw_description parameter is set in the call to help_formatter().

Author: grondo

src/bindings/python/flux/util.py

@@ -123,7 +123,7 @@ def encode_topic(topic):
     return topic
 
 
-def help_formatter(argwidth=40):
+def help_formatter(argwidth=40, raw_description=False):
     """
     Return our 'clean' HelpFormatter, if possible, with a wider default
      for the max width allowed for options.
@@ -162,6 +162,16 @@ def _format_action_invocation(self, action):
             args_string = self._format_args(action, default)
             return optstring + "=" + args_string
 
+    class FluxRawDescriptionHelpFormatter(
+        FluxHelpFormatter, argparse.RawDescriptionHelpFormatter
+    ):
+        pass
+
+    if raw_description:
+        return lambda prog: FluxRawDescriptionHelpFormatter(
+            prog, max_help_position=argwidth
+        )
+
     return lambda prog: FluxHelpFormatter(prog, max_help_position=argwidth)