sshfs: make _finalize method static to allow garbage collection

[skshetry]

Using a bound method with weakref.finalize was keeping a reference to the SSHFileSystem instance, preventing it from being garbage collected.

sshfs/sshfs/spec.py

Lines 73 to 75 in e371f39

	weakref.finalize(
	self, sync, self.loop, self._finalize, self._pool, self._stack
	)

sshfs/sshfs/spec.py

Line 98 in 32addf6

async def _finalize(self, pool, stack):

See note at the end of the Python documentation for weakref.finalize:

Note: It is important to ensure that func, args and kwargs do not own any references to obj,
either directly or indirectly, since otherwise obj will never be garbage collected.
In particular, func should not be a bound method of obj.

[shcheklein]

thanks!