forget to add 3.9 in pull_request.yml

Author: dleskosky

.github/workflows/pull_request.yml

@@ -9,7 +9,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.7, 3.8, 3.9]
         os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps: