Mark flake8-2020 as complete (#8902)

stubs/flake8-2020/METADATA.toml

@@ -1 +1,4 @@
 version = "1.7.*"
+
+[tool.stubtest]
+ignore_missing_stub = false

stubs/flake8-2020/flake8_2020.pyi

@@ -6,10 +6,21 @@ import ast
 from collections.abc import Generator
 from typing import Any, ClassVar
 
+YTT101: str
+YTT102: str
+YTT103: str
+YTT201: str
+YTT202: str
+YTT203: str
+YTT204: str
+YTT301: str
+YTT302: str
+YTT303: str
+
+class Visitor(ast.NodeVisitor): ...
+
 class Plugin:
     name: ClassVar[str]
     version: ClassVar[str]
     def __init__(self, tree: ast.AST) -> 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)