Bump flake8 from 3.7.9 to 3.8.2 in /src/requirements (#544)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.9 to 3.8.2.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.9...3.8.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

src/requirements/dev.txt

@@ -7,5 +7,5 @@ codecov==2.0.22
 black==19.10b0
 pre-commit==2.4.0
 isort==4.3.21
-flake8==3.7.9
+flake8==3.8.2