tests: refact flake8 workflow

drop ricardochaves/python-lint action and use `run` steps instead.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit f2d3432cad)

# Conflicts:
#	.github/workflows/flake8.yml

.github/workflows/flake8.yml

@@ -0,0 +1,18 @@
+name: flake8
+on: [pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [ '3.6', '3.7', '3.8' ]
+    name: Python ${{ matrix.python-version }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup python
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+          architecture: x64
+      - run: pip install flake8
+      - run: flake8 --max-line-length 160 ./library/ ./tests/library/