Reduce complexity found by Ruff

Simplify the case for lists and tuples

Author: tomschr

src/docbuild/config/merge.py

@@ -2,7 +2,7 @@
 
 from collections.abc import Mapping
 from copy import deepcopy
-from typing import Any, cast
+from typing import Any
 
 
 def deep_merge(*dcts: Mapping[str, Any]) -> dict[str, Any]:
@@ -53,13 +53,10 @@ def deep_merge(*dcts: Mapping[str, Any]) -> dict[str, Any]:
                     stack.append((existing, value))
 
                 # Lists / tuples → concatenate
-                elif isinstance(existing, (list, tuple)) and isinstance(
-                    value, (list, tuple)
+                elif (isinstance(existing, list) and isinstance(value, list)) or (
+                    isinstance(existing, tuple) and isinstance(value, tuple)
                 ):
-                    if isinstance(existing, list):
-                        dest[key] = existing + list(deepcopy(value))
-                    else:
-                        dest[key] = tuple(existing) + tuple(deepcopy(value))
+                    dest[key] = existing + deepcopy(value)  # type: ignore[operator]
 
                 # Sets → union
                 elif isinstance(existing, set) and isinstance(value, set):