Fix CI

Author: curusarn

.github/workflows/main.yml

@@ -6,7 +6,10 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # As long as we need Python 2.7 here in the test, we can only use up to Ubuntu 20.
+    # https://github.com/rwth-i6/returnn/issues/1226
+    # 20.04 is a LTS with 2030 support EOL 
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ['2.7', '3.5', '3.6', '3.7', '3.8', '3.9']