Use flake8 directly instead of unmaintained pytest plugin

The plugin isn't compatible to newer flake8 versions but doesn't seem to
be maintained. So just directly call flake8.

.flake8

@@ -0,0 +1,6 @@
+[flake8]
+per-file-ignores =
+    */migrations/* ALL
+    */tests/* ALL
+    __init__.py F401 F403
+max-line-length = 109

.github/workflows/ci.yml

@@ -20,6 +20,8 @@ jobs:
         pip install -r requirements-tests.txt
     - name: Run unit tests
       run: ./runtests.py --no-cov --no-vici
+    - name: Run flake8
+      run: flake8 strongMan/
 
   docker:
     runs-on: ubuntu-latest

pytest.ini

@@ -1,11 +1,5 @@
 [pytest]
-addopts = --tb=short --flake8
+addopts = --tb=short
 filterwarnings =
     error
-    ignore:SelectableGroups dict interface is deprecated::flake8
     ignore:'cgi' is deprecated and slated for removal in Python 3.13:DeprecationWarning
-flake8-ignore =
-    */migrations/* ALL
-    */tests/* ALL
-    __init__.py F401 F403
-flake8-max-line-length = 109

requirements-tests.txt

@@ -1,5 +1,5 @@
 -r requirements.txt
+flake8
 pytest==7.4.2
 pytest-django==4.1.0
-pytest-flake8==1.0.7
 coverage==5.3.1