Rename redo/python -> redo/py.

This avoids a name overlap with the system-installed copy of python.
Since redo adds the redo/ dir to the $PATH before running .do files,
python.do might see its own previously-created target instead of the
"real" python when testing, and create an infinite loop by accident.

redo/py.do

@@ -0,0 +1,7 @@
+redo-ifchange whichpython
+read py <whichpython
+cat >$3 <<-EOF
+	#!/bin/sh
+	exec $py "\$@"
+EOF
+chmod a+x $3