Ensure python standard argparse module is loaded

When 'import argparse' is used in cliff.something module,
cliff.argparse is loaded instead of python standard argparse.
This causes cliff.sphinxext failure with no attribute _HelpAction.

This can be avoided by renaming cliff.argparse to non-overlapping
module name like _argparse. cliff.argparse is for internal use,
so I believe it is safe to rename the module.

Closes-Bug: #1692018
Change-Id: Ic34f61581316c662dd2d672bc2c088dcef2100a9

cliff/app.py

@@ -22,7 +22,7 @@ import os
 import six
 import sys
 
-from cliff import argparse
+from cliff import _argparse
 from . import complete
 from . import help
 from . import utils
@@ -143,7 +143,7 @@ class App(object):
         :paramtype extra_kwargs: dict
         """
         argparse_kwargs = argparse_kwargs or {}
-        parser = argparse.ArgumentParser(
+        parser = _argparse.ArgumentParser(
             description=description,
             add_help=False,
             **argparse_kwargs

cliff/command.py

@@ -15,7 +15,7 @@ import inspect
 
 import six
 
-from cliff import argparse
+from cliff import _argparse
 
 
 @six.add_metaclass(abc.ABCMeta)
@@ -67,7 +67,7 @@ class Command(object):
     def get_parser(self, prog_name):
         """Return an :class:`argparse.ArgumentParser`.
         """
-        parser = argparse.ArgumentParser(
+        parser = _argparse.ArgumentParser(
             description=self.get_description(),
             epilog=self.get_epilog(),
             prog=prog_name,
@@ -97,7 +97,7 @@ class Command(object):
         return self.take_action(parsed_args) or 0
 
 
-class _SmartHelpFormatter(argparse.HelpFormatter):
+class _SmartHelpFormatter(_argparse.HelpFormatter):
     """Smart help formatter to output raw help message if help contain \n.
 
     Some command help messages maybe have multiple line content, the built-in