Update flake8 requirement from ~=3.8.4 to ~=3.9.0 (#62)

Updates the requirements on [flake8](https://gitlab.com/pycqa/flake8) to permit the latest version.
- [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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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-dev.txt

@@ -8,7 +8,7 @@
 # It only exists to keep the list of dependencies in a separate file from
 # setup.py.
 coverage ~=5.5
-flake8 ~=3.8.4
+flake8 ~=3.9.0
 pytest ~=6.2.2
 ipython ~=7.22.0
 numpydoc ~=1.1.0