Bump flake8 from 3.7.5 to 3.7.6 (#74)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.5 to 3.7.6.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.5...3.7.6

)

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

requirements.dev.txt

-flake8==3.7.5
+flake8==3.7.6
 Cython==0.29.5
 nose==1.3.7
 pylint==1.9.4