gdb/python: return gdbpy_ref<> from gdbpy_create_ptid_object

Update gdbpy_create_ptid_object (python/py-infthread.c) to return a
gdbpy_ref<> rather than a 'PyObject *'.  This reduces the chances that
a caller will leak an object, though no such memory leaks are fixed in
this commit, this is just a code improvement patch.

There should be no user visible changes after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>