FIX macos: Use the agg buffer_rgba rather than private attribute

The _renderer attribute has changed with the pybind11 update and
we aren't able to access it as a buffer anymore. We can use the
buffer_rgba method to get the direct memoryview of the buffer
instead.

Author: greglucas

src/_macosx.m

@@ -1236,7 +1236,7 @@ -(void)drawRect:(NSRect)rect
     CGContextRef cr = [[NSGraphicsContext currentContext] CGContext];
 
     if (!(renderer = PyObject_CallMethod(canvas, "get_renderer", ""))
-        || !(renderer_buffer = PyObject_GetAttrString(renderer, "_renderer"))) {
+        || !(renderer_buffer = PyObject_CallMethod(renderer, "buffer_rgba", ""))) {
         PyErr_Print();
         goto exit;
     }