provisional new installer variant for macOS 10.9 and later systems with 64-bit (x86_64)

I'd like to make a gentle request for this to be the primary, recommended installer so that the extension ecosystem can definitively standardize on targeting the 10.9 SDK for the 3.7 release cycle. Standardizing on 10.9 would:

• end the libc++/libstdc++ ABI (in)compatibility issues for extensions using C++
• dramatically reduce complication for library authors (e.g. Google's tensorflow won't support 10.6 -- the average library developer community has far fewer resources. Also many C++ packages are now requiring C++11 support, which the 10.6 SDK does not)

A quick look at some stats (1, 2) hints that the share of macOS users on versions < 10.9 is well under 10%, possibly in low single-digits. Though I suppose the most-relevant number may be python.org visitors -- they appear to use google analytics, which provides a breakdown by macOS version...