Flake8-logging

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* Add rule LOG009 that detects use of the undocumented ``WARN`` constant.

* Add rule LOG010 that detects passing calls to ``exception()`` passing a handled exception as the first argument.

* Add rule LOG011 that detects pre-formatted log messages.

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* LOG001: Avoid detecting inside function definitions when using ``Logger`` directly.

* Add rule LOG005 that recomends ``exception()`` over ``error()`` within ``except`` clauses.

* Add rule LOG006 that detects redundant ``exc_info`` arguments in calls to ``exception()``.

* Add rule LOG007 that detects ``exception()`` calls with falsy ``exc_info`` arguments, which are better written as ``error()``.

* Add rule LOG008 that detects calls to the deprecated, undocumented ``warn()`` method.

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* Change error codes to start with ``LOG`` so they are more specific.

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* Correct entry point definition so codes are picked up by default.

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* Initial release.