Mark ImageFile._open() as abstract

[radarhere]

Pillow/src/PIL/ImageFile.py

Lines 175 to 176 in 1cdb9a4

	def _open(self) -> None:
	pass

must be implemented in an ImageFile subclass.

Without this change,

from PIL import ImageFile
import io

class DummyImageFile(ImageFile.ImageFile):
  pass

DummyImageFile(io.BytesIO(b""))

gives

Traceback (most recent call last):
  File "demo.py", line 7, in <module>
    DummyImageFile(io.BytesIO(b""))
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "PIL/ImageFile.py", line 166, in __init__
    raise SyntaxError(msg)
SyntaxError: not identified by this driver

With this change, there is the much clearer

Traceback (most recent call last):
  File "demo.py", line 7, in <module>
    DummyImageFile(io.BytesIO(b""))
TypeError: Can't instantiate abstract class DummyImageFile with abstract method _open