Cachetools

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- Add submodule shims for backward compatibility.

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- Add documentation and tests for using ``TTLCache`` with
``datetime``.

- Link to typeshed typing stubs.

- Flatten package file hierarchy.

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- Update build environment.

- Remove Python 2 remnants.

- Format code with Black.

===================

- Handle ``__missing__()`` not storing cache items.

- Clean up ``__missing__()`` example.

===================

- Add FIFO cache implementation.

- Add MRU cache implementation.

- Improve behavior of decorators in case of race conditions.

- Improve documentation regarding mutability of caches values and use
of key functions with decorators.

- Officially support Python 3.9.

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- Improve ``popitem()`` exception context handling.

- Replace ``float('inf')`` with ``math.inf``.

- Improve "envkey" documentation example.