Implement io.IOBase.seekable method

Author: mxmlnkn

Diff for: sshfs/file.py

@@ -81,6 +81,9 @@ async def _open_file(self):
     def readable(self):
         return "r" in self.mode
 
+    def seekable(self):
+        return "r" in self.mode or "w" in self.mode
+
     def writable(self):
         return not self.readable()