Fix type annotations in user partitioning code

Context

When ./common/lib/xmodule/xmodule was moved to ./xmodule, we gained some increased fidelity on type errors in xmodule-related code, since mypy became able to locate ./xmodule files as local code (before, xmodule was treated as an external library).

One result of this was that a new mypy failure was introduced on this line. In particular, it seems that the enrollment_track_groups dictionary is annotated as a dictionary with str keys, whereas the keys can actually be of several different types, including str, int and NoneType. For the purpose of the common/lib/xmodule refactor PR, we opted to ignore the error, as fixing it would require fixing up several type annotations in https://github.com/openedx/edx-platform/tree/master/xmodule/partitions.

Acceptance Criteria

• In edx-platform's enrollment_track_partition_groups.py, on the line self.user_group = self.enrollment_track_groups.get(ENROLLMENT_TRACK_PARTITION_ID) # type: ignore, remove the # type: ignore.
• Fix any resulting cascade of type errors (by running make check-types or mypy path/to/file(s))

Start in openedx/core/djangoapps/content/learning_sequences/api/processors/enrollment_track_partition_groups.py at the self.user_group assignment. Remove the type-ignore noted in the acceptance criteria, then run make check-types or mypy on the affected files and fix any resulting cascade of type errors in the enrollment track and xmodule/partitions annotations. Done means the ignore is removed and the type checks pass.