bindings/python: rename Wrapper._clear()

Allow future.pimpl._clear() to be called from other parts
of the binding by dropping an underscore from its prefix.

The _clear method has some extra protections to ensure the C
destructor is only called once, thus future.pimpl._clear()
is the preferred way to explicitly dispose of a future.

This is a prereq for explicit cleanup of futures in the
job() class.

Originally proposed in #2553.

src/bindings/python/flux/wrapper.py

@@ -326,7 +326,7 @@ def __getattr__(self, name):
         setattr(self, name, new_method)
         return new_method
 
-    def __clear(self):
+    def _clear(self):
         # avoid recursion
         if hasattr(self, "_handle") and self._handle is not None:
             handle = self._handle
@@ -354,14 +354,14 @@ def handle(self, h):
                     )
                 )
         if self._handle is not None:
-            self.__clear()
+            self._clear()
         self._handle = h
 
     def __del__(self):
-        self.__clear()
+        self._clear()
 
     def __enter__(self):
         return self
 
     def __exit__(self, type_arg, value, unused):
-        self.__clear()
+        self._clear()