fix error running with 3.6

.github/workflows/push-check.yml

@@ -18,6 +18,9 @@ env:
 jobs:
   build-vsix:
     name: Create VSIX
+    # In order to use Python 3.6 here in the test, we can only use up to Ubuntu 20.
+    # https://github.com/rwth-i6/returnn/issues/1226
+    runs-on: ubuntu-20.04
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -30,7 +33,7 @@ jobs:
 
   lint:
     name: Lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -60,7 +63,7 @@ jobs:
 
       # Install bundled libs using 3.6 even though you test it on other versions.
       - name: Use Python 3.6
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v3
         with:
           python-version: '3.6'
 
@@ -86,7 +89,7 @@ jobs:
       # Now that the bundle is installed to target using python 3.7
       # switch back the python we want to test with
       - name: Use Python ${{ matrix.python }}
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v3
         with:
           python-version: ${{ matrix.python }}