Pull 'flake8-return' from source for unreleased fix

Prada Sarasola, Miguelrequested to merge
from-source-flake8-return into master

A fix to the problem described in #75 (closed) was merged to the main flake8-return repository a while ago, but it's not clear if/when a new version will be pushed to PYPI.

This fixes the errors by pointing to the source repository instead of PYPI (similar solution as !111 (merged), but without needing our own fork of the source). Ideally we should revert it once the new version is released.

Fixes #75 (closed).

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