While this looks similar to Perl's format strings, it is only
valid when the string is the target of 'format' method, like
this:

  _("foo {bar} baz").format(bar="bar")

Since the current xgettext Python parser scans tokens from left
to right, we transform the sequence into:

  .format(_("foo {bar} baz")).format(bar="bar")

and then scan the sequence again, so the flag context introduced by
'format' have an effect on the inner context.