workflows: test against 1 python version only

Let's drop py3.6 and py3.7

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit d191ba3)

.github/workflows/ansible-lint.yml

@@ -3,16 +3,12 @@ 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 }}
+          python-version: '3.8'
           architecture: x64
       - run: pip install -r <(grep ansible tests/requirements.txt) ansible-lint==4.3.7 netaddr
       - run: ansible-lint -x 106,204,205,208 -v --force-color ./roles/*/ ./infrastructure-playbooks/*.yml site-container.yml.sample site-container.yml.sample