[Python-3000] should Python 3's executable install as 'python'?

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    Martijn> "If you install Python 3.x on a system that already uses Python
    Martijn> 2.x, we encourage you to use 'make altinstall'. Doing "make
    Martijn> install" risks breaking existing scripts until they have been
    Martijn> updated to work with Python 3.0."

Of course the same holds true for people who install multiple minor
revisions of Python 2.x.  The README files on both trunk and the py3k branch
need updating.  If I get a chance today and nobody beats me to it I will
modify it with something like Oleg's wording.

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