Switch to ruff instead of flake8. (#14633)

ruff is a flake8-compatible Python linter written in Rust.
It supports the flake8 plugins that we use and is significantly
faster in testing.

stubs/frozendict.pyi

@@ -14,6 +14,8 @@
 
 # Stub for frozendict.
 
+from __future__ import annotations
+
 from typing import Any, Hashable, Iterable, Iterator, Mapping, Tuple, TypeVar, overload
 
 _KT = TypeVar("_KT", bound=Hashable)  # Key type.

stubs/icu.pyi

@@ -14,6 +14,8 @@
 
 # Stub for PyICU.
 
+from __future__ import annotations
+
 class Locale:
     @staticmethod
     def getDefault() -> Locale: ...

stubs/sortedcontainers/sorteddict.pyi

@@ -2,6 +2,8 @@
 # https://github.com/grantjenks/python-sortedcontainers/blob/eea42df1f7bad2792e8da77335ff888f04b9e5ae/sortedcontainers/sorteddict.pyi
 # (from https://github.com/grantjenks/python-sortedcontainers/pull/107)
 
+from __future__ import annotations
+
 from typing import (
     Any,
     Callable,

stubs/sortedcontainers/sortedlist.pyi

@@ -2,6 +2,8 @@
 # https://github.com/grantjenks/python-sortedcontainers/blob/a419ffbd2b1c935b09f11f0971696e537fd0c510/sortedcontainers/sortedlist.pyi
 # (from https://github.com/grantjenks/python-sortedcontainers/pull/107)
 
+from __future__ import annotations
+
 from typing import (
     Any,
     Callable,

stubs/sortedcontainers/sortedset.pyi

@@ -2,6 +2,8 @@
 # https://github.com/grantjenks/python-sortedcontainers/blob/d0a225d7fd0fb4c54532b8798af3cbeebf97e2d5/sortedcontainers/sortedset.pyi
 # (from https://github.com/grantjenks/python-sortedcontainers/pull/107)
 
+from __future__ import annotations
+
 from typing import (
     AbstractSet,
     Any,