FIX: Correct variable name from _frame to _frames in PillowWriter class

Author: Anselmoo

lib/matplotlib/animation.py

@@ -496,7 +496,7 @@ def grab_frame(self, **savefig_kwargs):
             "RGBA", self.frame_size, buf.getbuffer(), "raw", "RGBA", 0, 1)
         if im.getextrema()[3][0] < 255:
             # This frame has transparency, so we'll just add it as is.
-            self._frame.append(im)
+            self._frames.append(im)
         else:
             # Without transparency, we switch to RGB mode, which converts to P mode a
             # little better if needed (specifically, this helps with GIF output.)