Packaging

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* Fix a regression in parsing requirements with whitespaces between the comma
separators.

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* Fix a regression in parsing requirements like ``foo (==4)``.

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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.