Replace sys.stdout with sys.stdout.buffer when saving

[radarhere]

Resolves #5436

https://pillow.readthedocs.io/en/stable/reference/ImageDraw.html#example-draw-a-gray-cross-over-an-image

from PIL import Image, ImageDraw

with Image.open("hopper.jpg") as im:

    draw = ImageDraw.Draw(im)
    draw.line((0, 0) + im.size, fill=128)
    draw.line((0, im.size[1], im.size[0], 0), fill=128)

    # write to stdout
    im.save(sys.stdout, "PNG")

However, running this example code gives

Traceback (most recent call last):
  File "<stdin>", line 6, in <module>
  File "PIL/Image.py", line 2172, in save
    save_handler(self, fp, filename)
  File "PIL/PngImagePlugin.py", line 1223, in _save
    fp.write(_MAGIC)
TypeError: write() argument must be str, not bytes

This PR suggests fixing this by changing im.save so that if it is passed sys.stdout (sys.stdout.mode is 'w'), then it is replaced with sys.stdout.buffer (sys.stdout.buffer.mode is 'wb').

It also changes the PSDraw() default from sys.stdout to sys.stdout.buffer, and simplifies EPS saving by abandoning the idea of wrapping fp just so that it can write strings.

[hugovk]

Looks good, one question:

The docs for sys.stdout includes this:

Note: To write or read binary data from/to the standard streams, use the underlying binary buffer object. For example, to write bytes to stdout, use sys.stdout.buffer.write(b'abc').

OK, good. It continues:

However, if you are writing a library (and do not control in which context its code will be executed), be aware that the standard streams may be replaced with file-like objects like io.StringIO which do not support the buffer attribute.

Is this something we need to accomodate?

[radarhere]

Nice catch. Sure, I've added a commit to allow for that.

[hugovk]

Thank you!