migrate from fuse-python to pyfuse3

fuse-python depends on libfuse2 which is being phased out from most
distros because FUSE 3 has been the stable upstream for nearly a decade
and distros don't want to ship two major versions of the same library
indefinitely. Because of this fuse-python is no longer packaged in
Debian.

Replace the usage of fuse-python with pyfuse3 which uses an inode-based
async API backed by trio.

Fixes: #5
Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>

Author: obbardc