github: use runs-on: ubuntu-20.04 where python 3.6 is required

Problem: The python linting job fails on ubuntu-latest because python
3.6 is no longer supported.

Use runs-on: ubuntu-20.04 in the python-lint job so that python 3.6
can still be used.

.github/workflows/main.yml

@@ -15,7 +15,7 @@ jobs:
 
   python-lint:
     name: python linting
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/setup-python@v4
       with: