Don't use PyLong_FromUnsignedLong

This changes gdb to avoid PyLong_FromUnsignedLong, preferring
gdb_py_object_from_ulongest instead.

gdb/ChangeLog
2020-09-15  Tom Tromey  <tromey@adacore.com>

	* python/python.c (gdbpy_parameter_value): Use
	gdb_py_object_from_ulongest.