fix: TypeError: 'float' object cannot be interpreted as an integer. (#71)

Cast frame duration as an int to avoid TypeError

Co-authored-by: Martin Korbel <git@blackserver.cz>

Author: fdintino

src/pillow_avif/AvifImagePlugin.py

@@ -287,7 +287,7 @@ def _save(im, fp, filename, save_all=False):
                 # Append the frame to the animation encoder
                 enc.add(
                     frame.tobytes("raw", rawmode),
-                    frame_duration,
+                    int(frame_duration),
                     frame.size[0],
                     frame.size[1],
                     rawmode,