Trigger tests workflow on pull requests.

Author: TkTech

.github/workflows/test.yml

@@ -1,6 +1,6 @@
 name: Tests
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build:
@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8, '3.9.0-alpha - 3.9.0', pypy3]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9, pypy3]
 
     steps:
       # Python needs to be setup before checkout to prevent files from being