Support fuse3.x

[sdelafond]

As per https://bugs.debian.org/1084422, fuse2.x will be going away in Debian soon, and fuse3.x support is long overdue anyway.

I won't be able to spend time on this in the near future, so at this point this bug is mostly for tracking purposes, but I'll of course happily take PRs.

[tomatotomata]

I was thinking of starting with a compatibility pass rather than changing the whole binding at once: trace the current libfuse 2 API usage in fuse.py, setup.py, and the tests, then identify the smallest fuse3-compatible layer and add a focused test for the first changed operation. If the project prefers a separate compatibility module or a version-gated path, I can follow that instead of breaking existing fuse2 users. Would that be a useful first slice for this issue? Let me know what you think.