Ace-sklearn-crfsuite

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* Forked from original repository by ACE lab
* migrated to scikitlearn new api
* migrated to pipenv

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* added ``sklearn_crfsuite.metrics.flat_recall_score``.

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* Properly close file descriptor in ``FileResource.cleanup``;
* declare Python 3.6 support, stop testing on Python 3.3.

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* Small formatting fixes.

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* scikit-learn dependency is now optional for sklearn_crfsuite;
it is required only when you use metrics and scorers;
* added ``metrics.flat_precision_score``.

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* Ignore more errors in ``FileResource.__del__``.