Drop support for Python 3.5 (#4675)

Python 3.5 EOL was on 2020-09-30.

stdlib/3/sys.pyi

@@ -2,7 +2,22 @@ import sys
 from builtins import object as _object
 from importlib.abc import MetaPathFinder, PathEntryFinder
 from types import FrameType, ModuleType, TracebackType
-from typing import Any, Callable, Dict, List, NoReturn, Optional, Sequence, TextIO, Tuple, Type, TypeVar, Union, overload
+from typing import (
+    Any,
+    AsyncGenerator,
+    Callable,
+    Dict,
+    List,
+    NoReturn,
+    Optional,
+    Sequence,
+    TextIO,
+    Tuple,
+    Type,
+    TypeVar,
+    Union,
+    overload,
+)
 
 _T = TypeVar("_T")
 
@@ -209,12 +224,11 @@ if sys.version_info >= (3, 8):
     def addaudithook(hook: Callable[[str, Tuple[Any, ...]], Any]) -> None: ...
     def audit(__event: str, *args: Any) -> None: ...
 
-if sys.version_info >= (3, 6):
-    from typing import AsyncGenerator
+_AsyncgenHook = Optional[Callable[[AsyncGenerator[Any, Any]], None]]
 
-    _AsyncgenHook = Optional[Callable[[AsyncGenerator[Any, Any]], None]]
-    class _asyncgen_hooks(Tuple[_AsyncgenHook, _AsyncgenHook]):
-        firstiter: _AsyncgenHook
-        finalizer: _AsyncgenHook
-    def get_asyncgen_hooks() -> _asyncgen_hooks: ...
-    def set_asyncgen_hooks(firstiter: _AsyncgenHook = ..., finalizer: _AsyncgenHook = ...) -> None: ...
+class _asyncgen_hooks(Tuple[_AsyncgenHook, _AsyncgenHook]):
+    firstiter: _AsyncgenHook
+    finalizer: _AsyncgenHook
+
+def get_asyncgen_hooks() -> _asyncgen_hooks: ...
+def set_asyncgen_hooks(firstiter: _AsyncgenHook = ..., finalizer: _AsyncgenHook = ...) -> None: ...