Use ABCMeta in classes with abstractmethod

[radarhere]

https://docs.python.org/3/library/abc.html#abc.abstractmethod

@abc.abstractmethod
A decorator indicating abstract methods.

Using this decorator requires that the class’s metaclass is ABCMeta or is derived from it.

This is true. In main,

>>> from PIL import Image
>>> Image.ImagePointHandler()
<PIL.Image.ImagePointHandler object at 0x1044f93d0>

but once this PR changes the class to inherit from abc.ABC,

>>> from PIL import Image
>>> Image.ImagePointHandler()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class ImagePointHandler with abstract method point

While I'm here, I'm also going to replace

Pillow/src/PIL/ImageFile.py

Lines 458 to 460 in 7552893

	def _open(self) -> None:
	msg = "StubImageFile subclass must implement _open"
	raise NotImplementedError(msg)

and

Pillow/src/PIL/ImageFile.py

Lines 474 to 477 in 7552893

	def _load(self) -> StubHandler | None:
	"""(Hook) Find actual image loader."""
	msg = "StubImageFile subclass must implement _load"
	raise NotImplementedError(msg)

with abstract methods.