Bump flake8 from 3.8.4 to 3.9.0 (#80)

* Bump flake8 from 3.8.4 to 3.9.0

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

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* rm poetry.lock

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Shota Imaki <shota.imaki.0801@gmail.com>

pyproject.toml

@@ -15,7 +15,7 @@ requests = "^2.23.0"
 pytest = "^6.1.2"
 pytest-cov = "^2.10.1"
 codecov = "^2.1.10"
-flake8 = "^3.8.4"
+flake8 = "^3.9.0"
 black = "^20.8b1"
 isort = "^5.6.4"