gh-75707: tarfile: Add optional open() argument "mtime" (GH-138117)

This makes it possible to set the gzip header mtime field without
overriding time.time(), making it useful when creating reproducible
archives.

* 📜🤖 Added by blurb_it.

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Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Ethan Furman <ethan@stoneleaf.us>