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This release changes pex requirement resolution behavior. Only stable requirements are resolved by
default now. The previous behavior that included pre-releases can be retained by passing `--pre` on
the pex command line or passing `allow_prereleases=True` via the API.
* Upgrade dependencies to modern version ranges. (352)
`352 <https://github.com/pantsbuild/pex/pull/352>`_
* Add support for controlling prerelease resolution. (350)
`350 <https://github.com/pantsbuild/pex/pull/350>`_
`Fixes 28 <https://github.com/pantsbuild/pex/issues/28>`_