Add stubs for flake8-2020 (#6555)

stubs/flake8-2020/METADATA.toml

@@ -0,0 +1 @@
+version = "1.6.*"

stubs/flake8-2020/flake8_2020.pyi

@@ -0,0 +1,14 @@
+# flake8-2020 has type annotations, but PEP 561 states:
+# This PEP does not support distributing typing information as part of module-only distributions or single-file modules within namespace packages.
+# Therefore typeshed is the best place.
+
+import ast
+from typing import Any, ClassVar, Generator, Type
+
+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)