Reduce manual reference counting in py-inferior.c

This patch changes py-inferior.c to use gdbpy_ref<> when possible,
reducing the amount of manual reference counting.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-07-10  Tom Tromey  <tromey@adacore.com>

	* python/python-internal.h (create_thread_object): Return
	gdbpy_ref.
	* python/py-infthread.c (create_thread_object): Return gdbpy_ref.
	* python/py-inferior.c (struct threadlist_entry): Add
	constructor.
	<thread_obj>: Now a gdbpy_ref.
	(thread_to_thread_object): Update.
	(add_thread_object): Use new.
	(delete_thread_object): Use delete.
	(infpy_threads): Update.
	(py_free_inferior): Update.  Construct "inf_obj" after acquiring
	GIL.