Stop reading when EPS line becomes too long

[radarhere]

Resolves #6877

Using the following code to check speed (based on test_timeout()),

import time
import pytest
from PIL import Image
Image.init()
with open("Tests/images/timeout-d675703545fee17acab56e5fec644c19979175de.eps", "rb") as f:
    start = time.time()
    with pytest.raises(Image.UnidentifiedImageError):
        with Image.open(f):
            pass
    print("Time taken", time.time() - start)

I get

Time taken 0.07753515243530273

In EpsImagePlugin, after each time fp.readline() is run, an error is raised if fp.readline().strip("\r\n") is longer than 255 characters. So if Pillow isn't going to use the rest of that data, we can stop reading when the line becomes too long.

Applying that change, the speed test gives

Time taken 0.0006549358367919922

That's 118 times faster. This should be sufficient to stop intermittent timeouts in our CIs.

[hugovk]

Thanks!

[radarhere]

This was later reverted by #6879