Windows build busted with missing \Python3\ directory

Opening for @Mikekgr.

The Python3 merge broke Windows due to a renaming of the installed python directory.

See for full details:  https://github.com/esp8266/Arduino/pull/6378#issuecomment-526284744

Either the python.zip file for Windows needs to be moved on the toolchain dir to \python3\ (simplest), or the platform.txt needs to not use {python} as the subdir.