Fix the license classifier in distribution to the "Unlicense"

[jond01]

The package is licensed under the "Unlicense", but the PyPI distribution uses the following classifier:
https://github.com/tox-dev/py-filelock/blob/579ccc3412f2b370d7be64a9616f5d01d1f2e676/setup.cfg#L14
It's better to be changed to the following classifier:

License :: OSI Approved :: The Unlicense (Unlicense)

[gaborbernat]

PR welcome.