Remove more uses of explicit reference counting in Python

Remove more uses of explicit reference counting in Python

This changes some more place in the Python code to use gdbpy_ref
rather than explicit reference counting.  While doing this I found a
latent bug in typy_fields_items -- it was not checking for errors in
one spot.  I also changed valpy_dealloc to use Py_XDECREF rather than
an explicit "if".

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

	* python/py-value.c (valpy_dealloc): Use Py_XDECREF.
	* python/py-type.c (typy_fields_items): Use gdbpy_ref.
	* python/py-progspace.c (pspy_set_printers): Use gdbpy_ref.
	(pspy_set_frame_filters, pspy_set_frame_unwinders)
	(pspy_set_type_printers): Likewise.
	* python/py-function.c (fnpy_init): Use gdbpy_ref.
	* python/py-cmd.c (cmdpy_init): Use gdbpy_ref.
	* python/py-objfile.c (objfpy_set_printers): Use gdbpy_ref.
	(objfpy_set_frame_filters, objfpy_set_frame_unwinders)
	(objfpy_set_type_printers): Likewise.