I'm guessing it's because python is not in your path.
There's an example path on the download page of the new tools.
Quote: set path=mingw\mingw64-w32\bin;mingw\mingw64-w32\opt\bin;mingw\mingw64-w32\Qt\bin; (32-bit) set path=mingw\mingw64-w64\bin;mingw\mingw64-w64\opt\bin;mingw\mingw64-w64\Qt\bin; (64-bit)