Cacheout

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- Add ``Cache.on_get`` callback hook. Thanks uncle-lv_!
- Add ``Cache.on_set`` callback hook. Thanks uncle-lv_!

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- Add cache statistics. Thanks uncle-lv_!
- Add ``Cache.get_ttl``. Thanks uncle-lv_!
- Add ``Cache.on_delete`` callback hook. Thanks uncle-lv_!
- Add support for Python 3.11 and 3.12.

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- Set minimum Python version to 3.7 in setup.cfg.

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- Add support for Python 3.10.
- Drop support for Python 3.6. Minimum supported version is 3.7.
- Clarify docs around TTL to make it explicit what time units it uses by default.

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- Minor optimization in ``Cache.get_many|delete_many``.

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- Add ``cache_key`` attribute to memoized functions that can be used to generate the cache key used for a given set of function arguments. Thanks johnbergvall_!
- Fix bug in ``Cache.full`` that would result in an exception if cache created with ``maxsize=None`` like ``Cache(maxsize=None)``. Thanks AllinolCP_!
- Fix bug in ``Cache.get_many`` that resulted in ``RuntimeError: OrderedDict mutated during iteration`` when cache keys expire during the ``get_many`` call.
- Remove ``default`` argument from ``Cache.get_many``. A default value on missing cache key was only ever returned if a list of keys was passed in and those keys happened to expire during the ``get_many`` call. **breaking change**