github workflows exclude os/python version combinations

Author: pierreEBU

.github/workflows/pythonpackage.yml

@@ -10,6 +10,19 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
+        exclude:
+          - os: macos-latest
+            python-version: 3.5
+          - os: windows-latest
+            python-version: 3.5
+          - os: ubuntu-latest
+            python-version: 3.5
+          - os: macos-latest
+            python-version: 3.6
+          - os: windows-latest
+            python-version: 3.6
+          - os: ubuntu-latest
+            python-version: 3.6
 
     runs-on: ${{matrix.os}}