Install Python 3.9 for license finder

.github/workflows/license-audit.yml

@@ -4,18 +4,16 @@ on: [push, pull_request]
 
 jobs:
   license-audit:
-    # TODO: a GH action update broke the 'ubuntu-latest' image
-    #       when it's fixed, we should switch back
-    runs-on: ubuntu-20.04
+    runs-on: 'ubuntu-latest'
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v2
 
     - name: Set up Python
-      uses: actions/setup-python@v5
+      uses: actions/setup-python@v2
       with:
-        # License Finder's Docker image uses Python 3.5
-        python-version: 3.5
+        # License Finder's Docker image uses Python 3.9
+        python-version: '3.9'
 
     - name: Fetch decisions.yml
       run: curl https://raw.githubusercontent.com/bugsnag/license-audit/master/config/decision_files/global.yml -o decisions.yml