CI: Run flake8

Unlike the test action, this *does* use the builtin virtual-env-based
setup-python, since we do not need any system-level python packages, so
using this (and install flake8 using pip) is easier and always results
in the latest flake8 version.

Flake8 is run under Python3.5, so it should error out when any
constructs are used that are not supported by 3.5 yet.

Author: matthijskooijman

.github/workflows/testing.yml

@@ -60,3 +60,20 @@ jobs:
       - name: Run tests
         run: |
           dbus-launch python${{ matrix.python-version }} -m unittest
+
+  flake8:
+    name: Run code linting and checks
+    runs-on: ubuntu-latest
+    steps:
+      - name: Prepare repo
+        uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v1
+        with:
+          python-version: '3.5'
+      - name: Install flake8
+        run: |
+          pip install flake8
+      - name: Run flake8
+        run: |
+          flake8 --count --show-source --statistics