[wip][feature] fusepy: Add FUSE 3 support

fusepy/fusepy #66, #67, #101
fusepy/fusepy #100

First test with ratarmount worked!

 - [ ] I only monkey-patched readdir and getattr. The other changed
   methods should also be adjusted and tested and maybe we can do
   better, e.g., by letting the caller decide which interface they want
   to implement with a member variable as flag! Or do it via inspection
   like in fusepy/fusepy#101, but the overhead
   might be killer.