ci: update Black version

.github/workflows/black.yml

@@ -2,9 +2,9 @@ name: Code style
 
 on:
   push:
-    branches: [main, features]
+    branches: [master, features]
   pull_request:
-    branches: [main, features]
+    branches: [master, features]
 
 jobs:
   lint:
@@ -16,4 +16,5 @@ jobs:
     - uses: actions/setup-python@v2
       with:
         python-version: 3.9
-    - uses: psf/black@20.8b1
+    - uses: psf/black@21.7b0
+