Any Python objects that are used by callbacks should have their reference count incremented

[Keithcat1]

On one hand, if I create a BASS channel sync or any object that our C function callback will expect to have access to, set it on a channel and then discard it, it's possible for the Python GC to garbage collect it. Then our callback we passed to BASS comes along, tries to get a reference to that sync object, but oops it's not there anymore. Being able to set syncs on a channel and then just forget them would be nice.
On the other hand, implementing this is kind of a pain. You can use the following Cython-defined macros to increment and decrement the reference count of an object.

from cpython.ref cimport PyObject, Py_DECREF, Py_INCREF

Increment the reference just before converting the PyObject* to void* in order to pass it as a userdata parameter to BASS, then decrement the reference count once we know BASS is done with it or the object will never be freed which causes a memory leak. Unfortunately BASS doesn't provide any way of knowing when a sync's userdata can be freed, so channels have to keep a list of the syncs applied to them or something and Py_DECREF them all along with any other objects owned by the channel when the channel is freed. The best way of knowing when a channel is freed that I know of is setting a BASS_SYNC_FREE on the channel, which can't be used on HSAMPLE channels. BASS_SampleGetChannel supports the BASS_SAMCHAN_STREAM flag which returns a HSTREAM which is the same as normal but it's a stream, supports more BASS features and has slightly lower performance compared to not using BASS_SAMCHAN_STREAM, so we could just make using BASS_SAMCHAN_STREAM the default I guess.