gh-137976: Explicitly exclude `localtime` from `available_timezones` (#138012)

* fix: available_timezones is reporting an invalid IANA zone name

* 📜🤖 Added by blurb_it.

* correct rst format for backticks

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Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Paul Ganssle <1377457+pganssle@users.noreply.github.com>