The dependency graph has to know whether a driver must be re-evaluated
every frame due to a dependency on the current frame number. For python
drivers it was using a heuristic based on searching for certain sub-
strings in the expression, notably including '('.
When the expression is actually evaluated using Python, this can't be
easily improved; however if the Simple Expression evaluator is used,
this check can be done precisely by accessing the parsed data.
Differential Revision: https://developer.blender.org/D6624
fc1f5bded4