Packaging

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* Fix a bug where ``packaging.requirements:Requirement`` was overly strict when
matching legacy requirements.

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* Add a function that implements the name canonicalization from PEP 503.

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* Implement requirement specifiers from PEP 508.

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* Relicense so that packaging is available under *either* the Apache License,
Version 2.0 or a 2 Clause BSD license.

* Support installation of packaging when only distutils is available.

* Fix ``==`` comparison when there is a prefix and a local version in play.
(:issue:`41`).

* Implement environment markers from PEP 508.

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* Normalize post-release spellings for rev/r prefixes. :issue:`35`

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* Fix an error where the arbitrary specifier (``===``) was not correctly
allowing pre-releases when it was being used.

* Expose the specifier and version parts through properties on the
``Specifier`` classes.

* Allow iterating over the ``SpecifierSet`` to get access to all of the
``Specifier`` instances.

* Allow testing if a version is contained within a specifier via the ``in``
operator.