Backport the new 10.9+ installer variant from 3.7 to 3.6 for 3.6.5:

The rationale is that, while the 10.6 variant is still upwards compatible on current systems, it is showing its age.  Apple has started deprecating 32-bit support on macOS and apparently plans to drop it entirely in an upcoming release.  Also this gives a way to finally provide batteries-included Tcl/Tk support (the single most-requested feature for the macOS installers) and move to Tcl/Tk 8.6.x which is not well supported on older macOS releases (ActiveState currently only supplies 8.6.x binaries for macOS 10.9+, for example).  For 3.6.5, the 10.9+ variant will be offered as an additional alternative to the traditional 10.6+ variant in earlier 3.6.x releases.  I expect that most Binary extension modules (including wheels) built for earlier versions of 3.6.x with the 10.6 variant should continue to work with either 3.6.5 variant without recompilation.  This perhaps will also make it easier to transition to the 10.9+ variant as the default for 3.7.0.

In addition, both installer variants have updated 3rd-party libraries:
OpenSSL 1.0.2m -> 1.0.2n
XZ 5.2.2 -> 5.2.3
SQLite 3.21.0 -> 3.22.0

The 10.6 variant now sets CC=gcc instead of CC=gcc-4.2 and does not search for the outdated 10.6 SDK.  The variant is built with the same compiler as before.  If necessary for extension module builds, the CC value can be overridden with the CC env variable and an SDK can be specified with the SDKROOT env variable (see man xcrun).  These minor changes should be transparent to nearly all users.