Renamed mixin .py files to .pym (fixes davidhalter/jedi-vim#41)

jedi/mixin/_weakref.pym

@@ -0,0 +1,8 @@
+def proxy(object, callback=None):
+    return object
+
+class ref():
+    def __init__(self, object, callback=None):
+        self.__object = object
+    def __call__(self):
+        return self.__object