ci: flake8 workflow made more strict again (and CONTRIBUTING.md updated.)

Author: fjosw

.github/workflows/flake8.yml

@@ -21,6 +21,6 @@ jobs:
       - name: flake8 Lint
         uses: py-actions/flake8@v2
         with:
-          ignore: "E501,E121,E123,E126,E226,E24,E704,W503,W504"
+          ignore: "E501,W504"
           exclude: "__init__.py, input/__init__.py"
           path: "pyerrors"

CONTRIBUTING.md

@@ -33,6 +33,6 @@ pytest --cov=pyerrors --cov-report html
 ```
 The linter `flake8` is executed with the command
 ```
-flake8 --extend-ignore=E501 --exclude=__init__.py pyerrors
+flake8 --ignore=E501,W504 --exclude=__init__.py pyerrors
 ```
 Please make sure that all tests are passed for a new pull requests.