Remove atomicwrites dependency, use stdlib tempfile + os.replace

[dhruvildarji]

Summary

• Replace the deprecated atomicwrites library with stdlib tempfile.NamedTemporaryFile + os.replace/os.link
• Remove atomicwrites from pyproject.toml dependencies and requirements_examples.txt
• Preserves atomic write safety: writes to a temp file in the same directory, then renames atomically

Approach

As suggested in #118, atomicwrites is deprecated and the author recommends using os.replace / os.rename instead. This PR:

1. First usage (writing attempt timestamp): Uses NamedTemporaryFile + os.replace for atomic overwrite
2. Second usage (writing downloaded data): Uses NamedTemporaryFile + os.replace (when overwrite=True) or os.link (when overwrite=False, which raises FileExistsError if the file already exists, matching the previous behavior)

Both patterns write to a temp file in the same directory (ensuring same filesystem) before atomically moving into place, which is safe for concurrent downloads.

Test plan

• Verified no remaining references to atomicwrites in the codebase
• The atomic write pattern (temp file + rename) is the same approach used previously in this codebase before atomicwrites was introduced

Fixes #118