Description

There are two changes in b2; the free-threaded one is something we already knew about, but also the macOS version has finally been bumped from 10.9 to 10.13. This breaks up the question: how should we deal with it? For the least change, we can manually force MACOSX_DEPLOYMENT_TARGET to 10.13 if we detect it set less than 10.13, including the default 10.9. The other option is we could set the minimum across the board to 10.13, but users can set it lower (except for Python 3.13+, where we'd still need the above clamping). Thoughts? 10.13 brings nearly complete C++17 support (completed in 10.14, stared in 10.12).

(Not at https://www.python.org/api/v2/downloads/release/?is_published=true yet, guessing it's a bit too soon to show up (https://www.python.org/api/v2/downloads/release/?is_published=true&name=Python%203.13.0b2 is empty)? Not listed on python.org yet unless you know where to look - https://www.python.org/downloads/release/python-3130b2/)

Followup to #1831.

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