Add back cachetools stubs

[Akuli]

These were previously removed in #6093

• Added FIFO and MRU caches. This means that there is a new class cachetools.FIFOCache, a new function cachetools.func.fifo_cache, a new module cachetools.fifo, and similarly for MRU.
• Cache-type-specific submodules, such as cachetools.fifo and cachetools.lru, are now deprecated. Each of them contains one class that is also available directly in cachetools, so from cachetools.lru import LRUCache is deprecated in favor of from cachetools import LRUCache.
• The timer argument of cachetools.func.ttl_cache now has the correct type. Previously it was a float, even though it should be a callable that returns a float.

This is probably easiest to review as a whole, not commit-by-commit. Cachetools is not a big package.

[srittau]

It seems cachetools 4.2.4 re-added the missing modules.