test: add flake8 dependency

See [1].

1. https://pypi.org/project/pylint/

Author: DifferentialOrange

requirements-test.txt

@@ -2,3 +2,4 @@ git+https://github.com/baztian/dbapi-compliance.git@ea7cb1b4#egg=dbapi-complianc
 pyyaml==6.0
 importlib-metadata >= 1.0 ; python_version < '3.8'
 pylint
+flake8