Bump flake8 from 3.8.4 to 3.9.0 in /glean-core/python (#1548)

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)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

glean-core/python/requirements_dev.txt

@@ -2,7 +2,7 @@ auditwheel==3.3.1
 black==20.8b1
 cffi==1.14.5
 coverage==5.5
-flake8==3.8.4
+flake8==3.9.0
 flake8-bugbear==21.3.2
 jsonschema==3.2.0
 mypy==0.812