Make audio/plane pure

av/audio/plane.pxd

@@ -2,7 +2,5 @@ from av.plane cimport Plane
 
 
 cdef class AudioPlane(Plane):
-
     cdef readonly size_t buffer_size
-
     cdef size_t _buffer_size(self)

av/audio/plane.py

@@ -0,0 +1,13 @@
+import cython
+from cython.cimports.av.audio.frame import AudioFrame
+
+
+@cython.cclass
+class AudioPlane(Plane):
+    def __cinit__(self, frame: AudioFrame, index: cython.int):
+        # Only the first linesize is ever populated, but it applies to every plane.
+        self.buffer_size = self.frame.ptr.linesize[0]
+
+    @cython.cfunc
+    def _buffer_size(self) -> cython.size_t:
+        return self.buffer_size