revisit some points and optional assignments

[PythonCHB]

We're a bit consistent with zero point and optional assignments. Some are in Canvas, and some aren't.

and the dict lab should probably be 3 points, rather than zero.

While we're at it -- points for mailroom:

first version should be 15 points (as now), then eash additional iteration should have points pertaining to how hard it is -- i.e. part 2 and 3 probably 5 or ten points, then the test version and the OO version maybe 15 again.