Add stubs for flake8-builtins (#6483)

stubs/flake8-builtins/METADATA.toml

@@ -0,0 +1 @@
+version = "1.5.*"

stubs/flake8-builtins/flake8_builtins.pyi

@@ -0,0 +1,10 @@
+import ast
+from typing import Any, Generator, Type
+
+class BuiltinsChecker:
+    name: str
+    version: str
+    def __init__(self, tree: ast.AST, filename: str) -> None: ...
+    def run(self) -> Generator[tuple[int, int, str, Type[Any]], None, None]: ...
+
+def __getattr__(name: str) -> Any: ...  # incomplete (other attributes are normally not accessed)