Open BMP image with black palette in mode 1

[radarhere]

Saving a P mode image with only black and white (in that order) and re-opening it

from PIL import Image
im = Image.new("P", (1, 1))
im.putpalette([0, 0, 0, 255, 255, 255])
im.save("out.bmp")

with Image.open("out.bmp") as reloaded:
  print(reloaded.mode)

gives an 1 mode image.

This is because of

Pillow/src/PIL/BmpImagePlugin.py

Lines 281 to 283 in 1219c27

	# ------- If all colors are gray, white or black, ditch palette
	if grayscale:
	self._mode = "1" if file_info["colors"] == 2 else "L"

However, 1 mode is only applied if there are exactly two colours. This should be two colours or less. Otherwise, removing white actually changes the image to a more complex mode.

from PIL import Image
im = Image.new("P", (1, 1))
im.putpalette([0, 0, 0])
im.save("out.bmp")

with Image.open("out.bmp") as reloaded:
  print(reloaded.mode)

currently gives mode L. This PR changes it to mode 1 as well.