Packaging2

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* **BACKWARDS INCOMPATIBLE** Refactor specifier support so that it can sanely
handle legacy specifiers as well as PEP 440 specifiers.

* **BACKWARDS INCOMPATIBLE** Move the specifier support out of
``packaging.version`` into ``packaging.specifiers``.

~~~~~~~~~~~~~~~~~

* Add prerelease support to ``Specifier``.
* Remove the ability to do ``item in Specifier()`` and replace it with
``Specifier().contains(item)`` in order to allow flags that signal if a
prerelease should be accepted or not.
* Add a method ``Specifier().filter()`` which will take an iterable and returns
an iterable with items that do not match the specifier filtered out.

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* Allow ``LegacyVersion`` and ``Version`` to be sorted together.
* Add ``packaging.version.parse()`` to enable easily parsing a version string
as either a ``Version`` or a ``LegacyVersion`` depending on it's PEP 440
validity.

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* Initial release.


.. _`master`: https://github.com/pypa/packaging/