Packaging

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* Fix a logic error that was causing inconsistent answers about whether or not
a pre-release was contained within a ``SpecifierSet`` or not.

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

* Add ``Version().base_version`` and ``LegacyVersion().base_version`` to make
it easy to get the public version without any pre or post release markers.

* Support the update to PEP 440 which removed the implied ``!=V.*`` when using
either ``>V`` or ``<V`` and which instead special cased the handling of
pre-releases, post-releases, and local versions when using ``>V`` or ``<V``.

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* Normalize release candidates as ``rc`` instead of ``c``.

* Expose the ``VERSION_PATTERN`` constant, a regular expression matching
a valid version.

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* Ensure that versions are normalized before comparison when used in a
specifier with a less than (``<``) or greater than (``>``) operator.

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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``.

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* 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.