Decoders and encoders subclass PyDecoder and PyEncoder

[radarhere]

register_decoder() and register_encoder() state that decoder/encoder are callables that return an object.

Pillow/src/PIL/Image.py

Lines 3527 to 3534 in 912a33f

	def register_decoder(name: str, decoder) -> None:
	"""
	Registers an image decoder. This function should not be
	used in application code.
	:param name: The name of the decoder
	:param decoder: A callable(mode, args) that returns an
	ImageFile.PyDecoder object

Pillow/src/PIL/Image.py

Lines 3541 to 3548 in 912a33f

	def register_encoder(name, encoder):
	"""
	Registers an image encoder. This function should not be
	used in application code.
	:param name: The name of the encoder
	:param encoder: A callable(mode, args) that returns an
	ImageFile.PyEncoder object

However, if you look at Pillow's source code, it is not a callable, it is just an object.

Pillow/src/PIL/BlpImagePlugin.py

Lines 472 to 476 in 912a33f

	Image.register_decoder("BLP1", BLP1Decoder)
	Image.register_decoder("BLP2", BLP2Decoder)
	Image.register_save(BlpImageFile.format, _save)
	Image.register_encoder("BLP", BLPEncoder)

Pillow/src/PIL/PpmImagePlugin.py

Lines 362 to 363 in 912a33f

	Image.register_decoder("ppm", PpmDecoder)
	Image.register_decoder("ppm_plain", PpmPlainDecoder)

Pillow/src/PIL/DdsImagePlugin.py

Line 569 in 912a33f

Image.register_decoder("dds_rgb", DdsRgbDecoder)

Pillow/src/PIL/SgiImagePlugin.py

Line 230 in 912a33f

Image.register_decoder("SGI16", SGI16Decoder)

This PR updates the docstrings, clarifies this by adding type hints, and updates the tests (which did treat the arguments as callables).