Use _accept check in WebP _open

[radarhere]

Adds the _accept check into WebP _open.

This is already done by some other plugins. For example,

Pillow/src/PIL/GifImagePlugin.py

Lines 102 to 108 in 7e4ca8b

	def _open(self) -> None:
	# Screen
	assert self.fp is not None
	s = self.fp.read(13)
	if not _accept(s):
	msg = "not a GIF file"
	raise SyntaxError(msg)