⬆️ Bump pyflakes from 3.0.1 to 3.1.0 in /shifter

Bumps [pyflakes](https://github.com/PyCQA/pyflakes) from 3.0.1 to 3.1.0.
- [Changelog](https://github.com/PyCQA/pyflakes/blob/main/NEWS.rst)
- [Commits](PyCQA/pyflakes@3.0.1...3.1.0)

---
updated-dependencies:
- dependency-name: pyflakes
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

shifter/requirements_dev.txt

@@ -1,4 +1,4 @@
 flake8==6.0.0
 mccabe==0.7.0
 pycodestyle==2.11.0
-pyflakes==3.0.1
+pyflakes==3.1.0