Update cachetools to 4.2.3

[pyup-bot]

This PR updates cachetools from 4.1.0 to 4.2.3.

Changelog

4.2.3

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

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

- Link to typeshed typing stubs.

- Flatten package file hierarchy.

4.2.2

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

- Update build environment.

- Remove Python 2 remnants.

- Format code with Black.

4.2.1

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

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

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

4.2.0

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

- 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.

4.1.1

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

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

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

- Improve "envkey" documentation example.

Links

• PyPI: https://pypi.org/project/cachetools
• Changelog: https://pyup.io/changelogs/cachetools/
• Repo: https://github.com/tkem/cachetools/

[pyup-bot]

Closing this in favor of #751