prefer functools.cache on python>=3.9

[jorenham]

Because according to docs:

Because it never needs to evict old values, this is smaller and faster than lru_cache() with a size limit.

On python < 3.9 it will keep on using functools.lru_cache

[jorenham]

That github code editor isn't the smartest eh?

[jorenham]

it should be fixed now

[jorenham]

I believe the CI failure is the same codecov issue that we ignored in #5 @csala

[csala]

I believe the CI failure is the same codecov issue that we ignored in #5 @csala

It is. Merging this now.