Improve reference counting in py-type.c

Improve reference counting in py-type.c

This improves the reference counting in py-type.c by using gdbpy_ref
and gdbpy_ref::new_reference in more places.

gdb/ChangeLog
2019-01-03  Tom Tromey  <tom@tromey.com>

	* python/py-type.c (convert_field): Use new_reference.  Return
	gdbpy_ref.
	(make_fielditem): Return gdbpy_ref.
	(typy_fields): Update.
	(typy_getitem): Update.
	(field_name): Return gdbpy_ref.  Use new_reference.
	(typy_iterator_iternext): Update.