Support cachetools 6.x

[stronk7]

Right now matrix-nio only works with cachetools 5.x (~=5.3). It would be great to make it also support cachetools 6.x (~=6.2 right now).

The request is because many other libraries out there (fastmcp, ...) have started to require it and current restriction here renders the dependencies incompatible.

Looking to the cachetools 6.0 release, it seems that there weren't many breaking changes:

• Requires Python 3.9 (matrix-nio supports Python 3.8).
• MRUCache cache removed (matrix-nio does not use it).

Link to release notes: https://github.com/tkem/cachetools/blob/master/CHANGELOG.rst#v600-2025-05-23

And that's the story... for your consideration, ciao :-)