Pillow no longer reads grayscale multi-frame gifs

[FirefoxMetzger]

The following breaks in pillow 9.0.1 but works fine if I downgrade to pillow 8.4.0. My guess is that the new loader that converts to RGB/RGBA when seeking doesn't account for the possibility of a grayscale GIF.

from PIL import Image
import numpy as np

img = Image.open("gray_test.gif")
im1 = np.asarray(img)  # just showing that the first frame works
img.seek(2)  # break

Stack Trace:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\Sebastian\AppData\Local\Programs\Python\Python38\lib\site-packages\PIL\GifImagePlugin.py", line 133, in seek
    self._seek(f)
  File "C:\Users\Sebastian\AppData\Local\Programs\Python\Python38\lib\site-packages\PIL\GifImagePlugin.py", line 151, in _seek
    self.load()
  File "C:\Users\Sebastian\AppData\Local\Programs\Python\Python38\lib\site-packages\PIL\ImageFile.py", line 264, in load
    self.load_end()
  File "C:\Users\Sebastian\AppData\Local\Programs\Python\Python38\lib\site-packages\PIL\GifImagePlugin.py", line 371, in load_end
    self.im.putpalettealpha(self._frame_transparency, 0)
ValueError: image has no palette

Picture for testing:

[radarhere]

I've created PR #6086 to resolve this.