github/workflows: run black also on pull requests

It's happening quite often to have CI broken on the main branch
because we only run `black` on merges, to prevent these problems,
let's run `black` or PR as well.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

.github/workflows/black.yaml

@@ -1,5 +1,6 @@
 on:
   push:
+  pull_request:
 
 jobs:
   black:
@@ -11,4 +12,4 @@ jobs:
         run: pip install black
 
       - name: Run black
-        run: black . --check
+        run: black . --check