[scripts] build.py checks for pypy, uses cpython if pypy missing
[scripts] build.py checks for pypy, uses cpython if pypy missing

moreati/pydgin,futurecore/pydgin,moreati/pydgin,futurecore/pydgin,futurecore/pydgin,cornell-brg/pydgin,cornell-brg/pydgin,moreati/pydgin,futurecore/pydgin,moreati/pydgin,cornell-brg/pydgin,cornell-brg/pydgin