py-cachelib: updated to 0.3.0

Version 0.3.0
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-  Optimize ``FileSystemCache`` pruning.
-  Fix a bug in ``FileSystemCache`` where entries would not be removed
    when the total was over the threshold, and the entry count would be
    lost.
-  ``FileSystemCache`` logs system-related exceptions.
-  Removal of expired entries in ``FileSystemCache`` is only triggered
    if the number of entries is over the ``threshhold`` when calling
    ``set``. ``get`` ``has`` still return ``None`` and ``False``
    respectively for expired entries, but will not remove the files. All
    removals happen at pruning time or explicitly with ``clear`` and
    ``delete``.

(adam)