Allow manual invocation of the test workflow

Author: roelderickx

.github/workflows/test.yml

@@ -1,10 +1,14 @@
 name: test
 
-on: [push]
+on:
+  push:
+
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
 
 jobs:
   testpy3:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu:20
 
     steps:
     - uses: actions/checkout@v3