Dataclass-builder

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* Add :code:`update` function to facilitate updating a dataclass or dataclass
builder from a partially completed dataclass builder.

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* Switch to reST docstrings and remove paramref requirement.
* Fix bug in dataclass builders that contains a field named fields.
* Fix return type annotation of the build method.
* Fix issue `12`_ which would cause sentinel values :code:`REQUIRED`,
:code:`OPTIONAL`, and :code:`MISSING` to become incomparable after
a :code:`copy` or :code:`deepcopy`.

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* Fix issue `9`_ relating to the :code:`dataclass_builder` factory function
failing to handle dataclasses which use types from the :code:`typing` module.
* Replace equality checks with identity checks where possible. This was not
only the intention of the checks but by using identity, compatibility with
types that have broken equality operators is restored.

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* Fix bug where the :code:`MISSING` constant was not exported at the package
level.

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* Make :code:`MISSING` constant comparable (True) with :code:`REQUIRED` and
:code:`OPTIONAL` constants.

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* Fixed trove classifiers.