MNT: (deps): Bump pyflakes from 2.3.0 to 2.3.1 in /ci
MNT: (deps): Bump pyflakes from 2.3.0 to 2.3.1 in /ci

Bumps [pyflakes](https://github.com/PyCQA/pyflakes) from 2.3.0 to 2.3.1.
- [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/2.3.0...2.3.1)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
Unidata/siphon