Fixes #1515

Description

This PR improves compatibility in environments where the working directory is read-only but the file isn't.

• atomic_writing fallback: If the parent directory is not writable but the file is, fall back to direct write instead of using atomic write with a temp file.
• Checkpoints: If the .ipynb_checkpoints directory is not writable, redirect checkpoints to a temp path:
  $TMPDIR/jupyter_checkpoints/<rel_path>.

This ensures writing and checkpointing continue to work smoothly in restricted or non-standard setups.