Fix up some pytest style issues

[akx]

This PR fixes tests to use pytest.raises and pytest.warns as context managers everywhere possible.

[radarhere]

Is there a link that you can provide, describing this pytest style preference?

[akx]

@radarhere Kind of:

(Pillow) ~/b/Pillow (main) $ git grep pytest.raises | grep with | wc -l
     505
(Pillow) ~/b/Pillow (main) $ git grep pytest.raises | grep -v with | wc -l
      18
(Pillow) ~/b/Pillow (main) $ git grep pytest.warns | grep with | wc -l
      48
(Pillow) ~/b/Pillow (main) $ git grep pytest.warns | grep -v with | wc -l
      27

IOW, Pillow's tests themselves overwhelmingly use with pytest.raises/with pytest.warns.

[hugovk]

And I find im.save(out, save_all=True, append_images=ims) cleaner/more readable and closer to normal use than a big list of parameters: UserWarning, im.save, out, save_all=True, append_images=ims.

[akx]

Could this be merged? CI is green across the board.