Bump pyflakes from 1.6.0 to 2.1.1

Bumps [pyflakes](https://github.com/PyCQA/pyflakes) from 1.6.0 to 2.1.1.
- [Release notes](https://github.com/PyCQA/pyflakes/releases)
- [Changelog](https://github.com/PyCQA/pyflakes/blob/master/NEWS.rst)
- [Commits](https://github.com/PyCQA/pyflakes/compare/1.6.0...2.1.1)

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

requirements-dev.txt

@@ -2,7 +2,7 @@ coverage
 mock
 nose
 pycodestyle==2.5.0
-pyflakes==1.6.0
+pyflakes==2.1.1
 readme_renderer[md]==24.0
 requests_mock
 twine==1.15.0