Codejail logs all calls to `safe_exec` that include a `slug`.
Provide a `slug` in the Python-safe-exec debug view so that
usages of it are logged (for debugging purposes).

The limits can be defined in settings.CODE_JAIL['limits_overrides'],
which is a dict mapping context ids (generally, course run keys)
to overrides to be applied to settings.CODE_JAIL['limits'].

This will allow us to temporarily alter the codejail limits
for certain contexts in order to enable, for example,
a certain course run's tasks to run longer while a large exam's
grades are recomputed.

TNL-7649

This will remove imports from __future__ that are no longer needed.

https://docs.python.org/3.5/library/2to3.html#2to3fixer-future

* run python modernize

* run isort