[ci] Added flake8 pre-commit check.

.flake8

+[flake8]
+; max-line-length: see also black setting in pyproject.toml
+max-line-length = 80
+show-source = True

.pre-commit-config.yaml

@@ -23,5 +23,13 @@ repos:
     hooks:
       - id: isort
         exclude: EXPERIMENTAL/.*
+  - repo: https://github.com/PyCQA/flake8
+    rev: "5.0.4"
+    hooks:
+      - id: flake8
+        exclude: EXPERIMENTAL/.*
+        additional_dependencies:
+          - flake8-bugbear
+          - flake8-comprehensions
 default_language_version:
   python: python3.9