Fix syntax-related PEP8 issues

This changeset resolves 8 PEP8 issues: It addresses the following
syntax-related PEP8 violations:
- E401 multiple imports on one line
- E502 the backslash is redundant between brackets
- W601 .has_key() is deprecated, use 'in'
- E711 comparison to None should be 'if cond is None:'
- E712 comparison to True should be 'if cond is True:' or 'if cond:'

I'll lower the violation threshold after #5925 is merged.
The full PEP8 error message here was `E712 comparison to True should be
'if cond is True:' or 'if cond:'` so it would appear that `== True` is
definitely incorrect, while both your suggestion and mine are
acceptable.

I agree that `if cond:` is the more elegant solution. I originally chose
`if cond is True:` to maintain full backward-compatibility...

In the off-chance that someone set `ENABLE_INSTRUCTOR_EMAIL` to a
non-boolean `Truthy` value like `ENABLE_INSTRUCTOR_EMAIL = 'Yes'`, then
the value of `bulk_email_enabled_globally` would now become `Truthy`
instead of being cast to `False`, after this change. Granted, this case
is _probably_ unlikely and anyone who did so is still _wrong_, but this
is an important consideration in dynamic languages. We're _probably_
alright here, but changing type/cast (especially for boolean gates) can
lead to some _nasty_ regressions, for which there's often never test
coverage.

Rant aside, I don't mind changing, so let me know :)