switch to the py3 version of sphinx; if i'm not mistaken this avoids
the need to build both python 2 and 3 before building cmake. (this is on
the path to building about a third of the ports tree).
tests done on aarch64 i386 amd64, and some deps, ports-gcc, USE_LLD=no
tested on amd64.
there may be some missing as my unpacked ports source is a little out of date
but this should catch the main things people might run into
the struct was reordered a second time in sysctl.h r1.192 to improve
compatibility but amd64 snapshot packages made it out before that happened
so the bumps are still needed
