[mod] #424: Introduce flake8 pre-commit hook, conf and job

Set to 101 line length
because there is quite a lot ending at 101 chars
In .flake8, because pyproject.toml is not yet supported

.flake8

+[flake8]
+max-line-length = 101
+inline-quotes = "

.gitlab-ci.yml

@@ -38,6 +38,14 @@ build:
     paths:
       - ${PRE_COMMIT_HOME}
 
+flake8:
+  extends:
+    - .code_changes
+    - .pre-commit
+  stage: checks
+  script:
+    - pre-commit run --all-files flake8
+
 format:
   extends:
     - .code_changes

.pre-commit-config.yaml

@@ -43,3 +43,7 @@ repos:
     rev: 0.7.14
     hooks:
       - id: mdformat
+- repo: https://github.com/PyCQA/flake8
+  rev: 4.0.1
+  hooks:
+      - id: flake8