Upgrade GH runner to 20.04

.github/workflows/tests.yml

@@ -4,18 +4,17 @@ on: [push, pull_request]
 
 jobs:
   test:
-
+    runs-on: ubuntu-20.04
-    runs-on: ubuntu-18.04
     strategy:
       max-parallel: 16
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.7", "3.8", "3.9"]
 
     steps:
       - name: Checkout ZeroNet
         uses: actions/checkout@v2
         with:
-        submodules: 'true'
+          submodules: "true"
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v1