sshfs: make _finalize method static to allow garbage collection

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

See note at the end of the Python documentation for [`weakref.finalize`][1]:

> 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.

[1]: https://docs.python.org/3/library/weakref.html#weakref.finalize

Author: skshetry

sshfs/spec.py

@@ -100,7 +100,8 @@ async def _connect(
 
     connect = sync_wrapper(_connect)
 
-    async def _finalize(self, pool, stack):
+    @staticmethod
+    async def _finalize(pool, stack):
         await pool.close()
 
         # If an error occurs while the SSHFile is trying to