Added "transparency" argument to EpsImagePlugin load()

[radarhere]

Resolves #5619

At the moment, EpsImagePlugin can load images at different scales like so

Pillow/Tests/test_file_eps.py

Lines 48 to 49 in 426b730

	with Image.open(FILE1) as image1_scale2:
	image1_scale2.load(scale=2)

This PR adds another argument, "transparency",

with Image.open(FILE1) as image12: 
     image1.load(transparency=True)

to switch from "ppmraw"

Pillow/src/PIL/EpsImagePlugin.py

Line 120 in 426b730

"-sDEVICE=ppmraw", # ppm driver

to "pngalpha", generating an RGBA image with a transparent background, instead of an RGB image with a white background.

[hugovk]

Let's also add this to release notes.