Running Python 2 and Python 3 on the same machine

[Paul Watson]

The migration strategy detailed in PEP 3000 using 2to3 is quite nice.
However, I am looking for suggestions for migrating to 3 while I still
have code that requires 2.

Since the source code is incompatible, I was expecting the Python
executable to have a new name such as 'python3' or for the default
source code filename to change to '.py3' or something.

Yes, I have searched some on the web and the newsgroup, so please don't
beat me up if this is well known.  Thanks.