Update integration-test.yml to Python 3.9 (pytorch#3862)

As we no longer build PyTorch for 3.8

Author: kit1980

.github/workflows/integration-test.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [ 3.8 ]
+        python-version: [ 3.9 ]
 
     steps:
     - uses: actions/checkout@v2