Keep transparency when converting from P to LA or PA

[radarhere]

Resolves #5593, fixing the second problem in that issue.

At the moment, there is code in convert() to keep transparency when going from P to RGBA.

Pillow/src/PIL/Image.py

Lines 1008 to 1017 in be30792

	elif self.mode == "P" and mode == "RGBA":
	t = self.info["transparency"]
	delete_trns = True
	if isinstance(t, bytes):
	self.im.putpalettealphas(t)
	elif isinstance(t, int):
	self.im.putpalettealpha(t, 0)
	else:
	raise ValueError("Transparency for P mode should be bytes or int")

This PR expands the condition to do the same if going from P to LA or PA as well.