* INCR-219: Run python-modernize on `openedx/core/djangoapps/safe_sessions`
* disable pylint on six import to avoid isort and pylint guideline conflict
An @contextmanager will raise an exception from its yield statement if
an exception happens in the with-block that uses it. If the context
manager needs to do clean up, it should do it even if an exception is
raised, so it needs to be done in a finally clause.
1. Use request.session instead of request.user, since request.user
won't necessarily be properly set.
2. Be extra paranoid by putting logging after session cookie deletion,
so that even if there is some error related to logging, the important
work will complete and the browser won't get left in a broken state.
3. Write out the full contents of the Cookie header (up to 4096 bytes)
in the log as a base64 encoded string. This way we can look at broken
cookie states and diagnose what's breaking them (the Python parser will
just silently skip anything past a corrupted cookie entry). We base64
encode mostly to prevent people from maliciously injecting garbage into
our logs.
