Request: Add test which fails if `self.evaluator = None` is removed

[adamtheturtle]

PythonDocument.evaluator in sybil/document.py includes the line self.evaluator = None.
mypy has issues with this line, and I am trying to resolve those issues.
However, I cannot be confident that my attempts are valid, as I don't understand what this is meant to do, and the tests pass if this line is removed.
My request is that a test is added which fails if this line is removed, so that I can have more confidence.

[cjw296]

This ends up going away in the branch I'm currently working on.

[adamtheturtle]

Thanks! I'll wait for that