In 09088690 we mistakenly added E501 to the flake8 ignore list. Since
then, many new violations have been introduced. This patch re-enables
the check and corrects all violations, except in some cases like unit
test names where the subunit output would suffer if we attempted to
shorten the function name.
This may appear to be a pointless no-op that messes with
git-blameability, and it is, but the reason to do this is that if PEP8
violations are introduced in master and then backported to a stable
branch, most stable branches will fail the pep8 job since the flake8
ignore list is correct for those branches. Rather than loosening the
check in older branches or requiring those backports to fix the linter
errors independently of what's been merged in master, we should fix it
now so that we don't introduce more errors in the future and patches can
more easily be backported.