pytype: only try the hacky way of finding PYTHON if not provided

This allows us to work in more environments, like when using pyenv. This
syntax is compatible with all POSIX shells.

contrib/setup-pytype.sh

@@ -5,7 +5,7 b' set -u'
 
 # Find the python3 setup that would run pytype
 PYTYPE=`which pytype`
-PYTHON3=`head -n1 ${PYTYPE} | sed -s 's/#!//'`
+PYTHON3=${PYTHON:-`head -n1 ${PYTYPE} | sed -s 's/#!//'`}
 
 # Existing stubs that pytype processes live here
 TYPESHED=$(${PYTHON3} -c "import pytype; print(pytype.__path__[0])")/typeshed/stubs