github: force python 3.8 for pylint

Pylint does not support python 3.9 properly:

  Value 'Optional' is unsubscriptable (unsubscriptable-object)

This is a known issue. Until it is fixed run the lint checks with python
3.8.

Link: pylint-dev/pylint#3882
Signed-off-by: Robin Jarry <robin.jarry@6wind.com>

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: 3.x
+          python-version: 3.8
       - uses: actions/cache@v2
         with:
           path: ~/.cache/pip