Proof

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* Use Python's logging module instead of ``print()``.
* Add support for recent Python versions.
* Drop support for end-of-life Python versions.

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* Fix errors when caching unicode analysis function names.
* Fix errors when caching unicode source. (10)
* Add six dependency.
* Add decorator and logic for analysis functions that are never cached. (5)

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* Don't load analysis cache unless needed. (1)

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* Initial code migration from agate.