Add new checker useless-return

[atodorov]

warns about "return" and "return None" statements at the end of functions/methods. These statements are useless because None is the default return type if they are missing.

Please comment on this feature because there are 20+ existing test cases that break with it. I'd like to know how to "fix" the failing tests before updating documentation, etc.

[PCManticore]

This contradicts with the new inconsistent-return-statements check that landed in 1.8: https://pylint.readthedocs.io/en/latest/whatsnew/1.8.html

[atodorov]

@PCManticore

the inconsistent-return-statements says

                  'According to PEP8, if any return statement returns an expression, '
                  'any return statements where no value is returned should explicitly '
                  'state this as return None, and an explicit return statement '
                  'should be present at the end of the function (if reachable)'

however that leaves out the case where there is only one return statement at the end of the function body and it is either "return" or "return None". I am currently working with a legacy codebase which has many of these. For example

def add_cases(run_ids, case_ids):
    """
    trs = TestRun.objects.filter(run_id__in=pre_process_ids(run_ids))
    tcs = TestCase.objects.filter(case_id__in=pre_process_ids(case_ids))

    for tr in trs.iterator():
        for tc in tcs.iterator():
            tr.add_case_run(case=tc)

    return

IMO this checker can still be beneficial to people, but probably needs to be better integrated with the inconsistent-return-statements checker. What do you think ?