Deprecate Image._show

[radarhere]

I suggest deprecating this method.

Pillow/src/PIL/Image.py

Lines 3793 to 3800 in 208e9b5

	# --------------------------------------------------------------------
	# Simple display support.
	def _show(image: Image, **options: Any) -> None:
	from . import ImageShow
	ImageShow.show(image, **options)

The comment used to read

Simple display support. User code may override this.

but that changed with #4713 in Pillow 7.2.0.