gh-154726: Fix shutil.copyfile() for symlinks to special files with follow_symlinks=False

[lkk7]

Fixes #154726.

• Avoid checking a source symlink's target when follow_symlinks=False
• Add a regression test using a symlink to /dev/null.

• Issue: shutil.copyfile() rejects device symlinks with follow_symlinks=False #154726

[aisk]

fifo / socket have the same problem, and I think they are already fixed by this PR. We should update the PR's title to mention this, like using 'special file' in the news entry.

And maybe we can update the tests to test socket and fifo to ensure this and prevent future regressions. But it's up to you.

[lkk7]

Hi @aisk, done.

I just noticed that one test is not grouped with the others, but maybe it's too cosmetic to run the whole CI again for that. If any other change is needed, then I'll push that too.