Use stable black, use python 3.9

.github/workflows/python_style_checks.yml

@@ -25,6 +25,10 @@ jobs:
     # The type of runner that the job will run on
     runs-on: ubuntu-latest
 
+    strategy:
+      matrix:
+        python-version: [3.9]
+
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
@@ -33,7 +37,7 @@ jobs:
       - uses: actions/setup-python@v2
 
       # Run black code formatter
-      - uses: psf/black@21.4b2
+      - uses: psf/black@stable
         with:
           args: ". --check"
 
@@ -42,7 +46,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.7]
+        python-version: [3.9]
 
     steps:
     - uses: actions/checkout@v2