gh-85567: Register a cleanup function to close files for FileType objects in argparse (#32257)

* bpo-41395: Register a cleanup function to close files for FileType objects in argparse

* Added import as top level import, and renamed file as fh.

Lib/argparse.py

@@ -84,7 +84,7 @@ __all__ = [
     'ZERO_OR_MORE',
 ]
 
-
+import atexit as _atexit
 import os as _os
 import re as _re
 import sys as _sys
@@ -1268,8 +1268,12 @@ class FileType(object):
 
         # all other arguments are used as file names
         try:
-            return open(string, self._mode, self._bufsize, self._encoding,
-                        self._errors)
+            fh = open(string, self._mode, self._bufsize, self._encoding, self._errors)
+
+            # Register cleanup function to close file
+            _atexit.register(fh.close)
+
+            return fh
         except OSError as e:
             args = {'filename': string, 'error': e}
             message = _("can't open '%(filename)s': %(error)s")