Use cache instead of lru_cache once minimum python version is >=3.9

[mscroggs]

#271 uses functools to cache the results of apply_restrictions. From Python 3.9 onwards, we can use functools.cacheinstead offunctools.lru_cache`.

Python 3.8 end of life is October 2024.

[mscroggs]

closing as it looks like we need custom caching to avoid memory leaks