Fix action on MacOS on GHA (attempt 2)

The current MacOS environment (Catalina 10.15) on GHA has python linked
to Python 2.7 ... so let's use python3 as the binary then!

action.yml

@@ -41,7 +41,7 @@ runs:
         fi
 
         envpath="${{ github.action_path }}/.black-venv"
-        python -m venv $envpath
+        python3 -m venv $envpath
         # Please don't ask me questions on why this specific form is necessary so the
         # output doesn't up mangled. I really don't know why, but I don't want to
         # dump even more hours on trying to understand it. BUT I will blame Windows