[Distutils] Downloading betas?

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I have been using ZODB 3.6.0 for SpamBayes for awhile.  I noticed today that
ZODB 3.8.0 is available from PyPI, so I ran

    easy_install -U ZODB3

It determined that 3.8.1b8 was the "best" match.  Why didn't it stop with
3.8.0 instead of installing a version which was clearly marked as beta?  Is
there a way to tell it, "don't do that"?

Thx,

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