INCR-411: Updates on Python 3.x

The `NoneType` object is not available from the `types` module in
python 3. This is a workaronud to deal with that.

Use `ascii_letters` instead which is a reasonable alternative for our
usecases.

add submission deadline methods in InheritanceMixin

INCR-443

INCR-445

INCR-380 python 3 compatibility

INCR-379 python 3 compatibility

INCR-377 python 3 compatibility

INCR-398

Fixed CDN urls to Static urls on edit.

INCR-397

INCR-387 Python 3 compatibility

INCR-382 Python 3 compatibility

INCR-395

INCR-394

INCR-376 python3 compatibility

INCR-375 python3 comptatibility

INCR-366 python3 compatibility

Make upgrade

* add pytest decorator for skipping py3 test

* quality

* Remove full table scan of VerificationDeadline.

Before this commit, we were doing a full table scan of
student_verificationdeadline, loading the results into a giant
dict, and reading/writing that to the cache. This was fine when the
code was introduced and there were dozens of courses, but now that
we're over 12K courses, it's becoming a major performance issue for
the Student Dashboard.

This uses a subquery to the course enrollment table so that we're
only ever pulling back the deadlines to a student's enrolled courses
for any given request. It removes the cache access entirely.