login: simplify login view logic for generating return_url

The old code checked request.method == "POST" twice, it can be simplified now to only check that once, and still do exactly the same thing.

src/sambal/views/auth.py

@@ -9,16 +9,9 @@
 @forbidden_view_config(accept="text/html", renderer="login.jinja2")
 def login(request):
     """Login form."""
-    # Avoid looping the login page if accessed directly.
-    # Because the app also uses traversal request.matched_route can be None.
     if request.method == "POST":
         return_url = request.POST.get("return_url", request.path)
-    elif request.matched_route and request.matched_route.name == "login":
-        return_url = request.route_path("home")
-    else:
-        return_url = request.path
 
-    if request.method == "POST":
         if (form := LoginForm(request.POST)) and form.validate():
             username = form.username.data
             password = form.password.data
@@ -35,6 +28,13 @@ def login(request):
                 msg = e.args[1]
                 request.session.flash(f"Login failed: {msg}", queue="error")
     else:
+        # Avoid looping the login page if accessed directly.
+        # Also, as the app uses traversal request.matched_route can be None.
+        if request.matched_route and request.matched_route.name == "login":
+            return_url = request.route_path("home")
+        else:
+            return_url = request.path
+
         form = LoginForm()
 
     return {