Update constructor.py

Changes 'collections.Hashable' to 'collections.abc.Hashable' to fix #128

rplibs/yaml/yaml_py3/constructor.py

@@ -123,7 +123,7 @@ def construct_mapping(self, node, deep=False):
         mapping = {}
         for key_node, value_node in node.value:
             key = self.construct_object(key_node, deep=deep)
-            if not isinstance(key, collections.Hashable):
+            if not isinstance(key, collections.abc.Hashable):
                 raise ConstructorError("while constructing a mapping", node.start_mark,
                         "found unhashable key", key_node.start_mark)
             value = self.construct_object(value_node, deep=deep)