chore: add flake8 pre-commit hook

Part-of: <!58>

.flake8

+# Use .flake8 file until pyproject.toml is supported
+# See https://github.com/PyCQA/flake8/issues/234
+[flake8]
+max-line-length = 120

.pre-commit-config.yaml

@@ -39,6 +39,11 @@ repos:
     hooks:
       - id: isort
 
+  - repo: https://github.com/pycqa/flake8
+    rev: 6.0.0
+    hooks:
+      - id: flake8
+
   - repo: https://github.com/pre-commit/mirrors-prettier
     rev: v2.7.1
     hooks: