Python: Update python-brace-format, assuming Python >= 3.1.

Python: Update python-brace-format, assuming Python >= 3.1.

* gettext-tools/src/format-python-brace.c (struct toplevel_counters): New type.
(parse_directive): Take a 'struct toplevel_counters *' parameter. Allow empty
argument names at the top-level. But reject format strings with both absolute
argument numbers and unnamed arguments at the top-level.
(parse_upto): Take a 'struct toplevel_counters *' parameter.
(format_parse): Pass a 'struct toplevel_counters *' to parse_upto.
* gettext-tools/tests/format-python-brace-1: Update.
* gettext-tools/tests/format-python-brace-2: Add another test case.
* gettext-tools/doc/lang-python.texi: Add more examples of formatting with
positions.