fixed Ubuntu version at 20.04

Author: yamatt

.github/workflows/python3.yml

@@ -15,7 +15,7 @@ on:
 jobs:
   lint:
     name: Run some linting
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}