Rename rtest.sh to rtest.py

The reason this is a separate commit from the next commit, which
contains the changed content, is to trick git to show the changes as a
one-file diff regardless of the similarity index, rather than one
deleted and one added file. This makes it easier to compare the ksh
code to the Python code.

diff --git a/rtest/rtest.sh b/rtest/rtest.py
similarity index 100%
rename from rtest/rtest.sh
rename to rtest/rtest.py