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.

t/101-atime/atime2.do

@@ -1,6 +1,6 @@
 # make sure redo doesn't think merely *reading* the old file counts as
 # modifying it in-place.
 cat $1 >/dev/null 2>/dev/null || true
-../../redo/python tick.py
+../../redo/py tick.py
 cat $1 >/dev/null 2>/dev/null || true
 echo hello