python.set kept the stored result whenever the code was unchanged, also when
the call switched the return type between an Excel value and a Python object.
The cell then showed a value and preview of the other type, and a save wrote
the new type with that old cached value. The result is now kept only when both
the code and the return type match; otherwise the cell is new code, which
shows #BLOCKED! until it runs, and its old spill goes with its readers
recalculated.