Use __truediv__ instead of __div__

This ignores Python 2, but that shouldn't be an issue, since we are going to drop it anyway.

jedi/inference/syntax_tree.py

@@ -35,7 +35,7 @@ operator_to_magic_method = {
     '+': '__add__',
     '-': '__sub__',
     '*': '__mul__',
-    '/': '__div__',
+    '/': '__truediv__',
     '//': '__floordiv__',
     '%': '__mod__',
     '**': '__pow__',