Log Message:
py-typing-extensions: Avoid circular dependency with Python 3.7

Switch to use NO_BUILD=yes with wheel.mk (and change MASTER_SITE_WHEEL
to use pre-built platform independent packages) in order to avoid a circular
dependency with Python 3.7 in py-build.

Bump PKGREVISION.

Thanks a lot to <mef> and <nros>! (possible left-over regressions \ 
are mine)