Log Message:
py-cachetools: updated to 4.2.1

v4.2.1 (2021-01-24)
===================
- Handle ``__missing__()`` not storing cache items.
- Clean up ``__missing__()`` example.

v4.2.0 (2020-12-10)
===================
- 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.