Fix argparse.py causing warnings when passed to xgettext.

This patch was already pushed to upstream and will is part of Python 3.8 and later.
https://github.com/python/cpython/pull/16080/
https://bugs.python.org/issue37785/

--- Python-3.7.5/Lib/argparse.py
+++ Python-3.7.5/Lib/argparse.py
@@ -1207,8 +1207,9 @@ class FileType(object):
             return open(string, self._mode, self._bufsize, self._encoding,
                         self._errors)
         except OSError as e:
-            message = _("can't open '%s': %s")
-            raise ArgumentTypeError(message % (string, e))
+            args = {'filename': string, 'error': e}
+            message = _("can't open '%(filename)s': %(error)s")
+            raise ArgumentTypeError(message % args)
 
     def __repr__(self):
         args = self._mode, self._bufsize