Packvers

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* Fix string representation of PEP 508 direct URL requirements with markers.

* Better handling of file URLs

This allows for using ``file:///absolute/path``, which was previously
prevented due to the missing ``netloc``.

This allows for all file URLs that ``urlunparse`` turns back into the
original URL to be valid.

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* Improve error messages when invalid requirements are given. (`129 <https://github.com/pypa/packaging/issues/129>`__)

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* Fix ``utils.canonicalize_version`` when supplying non PEP 440 versions.

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* Drop support for python 2.6, 3.2, and 3.3.

* Define minimal pyparsing version to 2.0.2 (`91 <https://github.com/pypa/packaging/issues/91>`__).

* Add ``epoch``, ``release``, ``pre``, ``dev``, and ``post`` attributes to
``Version`` and ``LegacyVersion`` (`34 <https://github.com/pypa/packaging/issues/34>`__).

* Add ``Version().is_devrelease`` and ``LegacyVersion().is_devrelease`` to
make it easy to determine if a release is a development release.

* Add ``utils.canonicalize_version`` to canonicalize version strings or
``Version`` instances (`121 <https://github.com/pypa/packaging/issues/121>`__).

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* Fix markers that utilize ``in`` so that they render correctly.

* Fix an erroneous test on Python RC releases.

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* Add support for the deprecated ``python_implementation`` marker which was
an undocumented setuptools marker in addition to the newer markers.