github: remove version from python-format action

Problem: The "python-format" GitHub action fails with:

"Error: The version '3.7' with architecture 'x64' was not found for
Ubuntu 24.04."

because Python 3.7 reached its EOL.

Fix the version of Ubuntu for the "python-format" action to
ubuntu-20.04 instead of using ubuntu-latest.

.github/workflows/main.yml

@@ -24,7 +24,7 @@ jobs:
 
   python-format:
     name: python format
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with: