xgettext: In language Python, fix a format string handling bug.
The option --flag=my_new_format:1:python-brace-format used to mark strings
inside my_new_format invocations as being 'python-format' instead of
'python-brace-format'.
* gettext-tools/src/xgettext.c (xgettext_record_flag): Use the correct index
into flag_table_python.
* gettext-tools/tests/xgettext-python-7: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
