Call _accept instead of duplicating code

[radarhere]

_open sometimes contains the same check as _accept. This PR changes some plugins to call _accept directly, instead of just duplicating the code.

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

Pillow/src/PIL/CurImagePlugin.py

Lines 38 to 45 in 2456601

	def _open(self):
	offset = self.fp.tell()
	# check magic
	s = self.fp.read(6)
	if not _accept(s):
	raise SyntaxError("not a CUR file")