OsOperations.isdir is added

It a correction of base interface - OsOperations.

It seems we forgot to add a declaration of the "abstract" method "isdir".

Author: dmitry-lipetsk

testgres/operations/os_ops.py

@@ -107,6 +107,9 @@ def read_binary(self, filename, start_pos):
     def isfile(self, remote_file):
         raise NotImplementedError()
 
+    def isdir(self, dirname):
+        raise NotImplementedError()
+
     def get_file_size(self, filename):
         raise NotImplementedError()