Finalize HIDAPI only when ref count reaches zero

Replace atexit.register() with weakref.finalize(), since that should
guarantee that there are no remaining references to the module (and,
thus, HIDAPI) when it runs.

Related: #128 ("Fatal Python error: Segmentation fault on 0.11.0.post2")

hid.pyx

@@ -1,5 +1,6 @@
 import sys
-import atexit
+import weakref
+
 from chid cimport *
 from libc.stddef cimport wchar_t, size_t
 
@@ -382,5 +383,6 @@ cdef class device:
         return U(<wchar_t*>hid_error(self._c_hid))
 
 
-# Set a callback to close the HID library as the script exits
-atexit.register(hidapi_exit)
+# Finalize the HIDAPI library *only* once there are no more references to this
+# module, and it is being garbage collected.
+weakref.finalize(sys.modules[__name__], hidapi_exit)