Add EnumMeta.__bool__ (#7206)

Most Python objects evaluate as falsey if they have length 0, but an enum class is truthy even if it has length 0.

Source code: https://github.com/python/cpython/blob/841c77d802e9ee8845fa3152700474021efe03fd/Lib/enum.py#L353

Author: AlexWaygood

stdlib/enum.pyi

@@ -5,6 +5,7 @@ from abc import ABCMeta
 from builtins import property as _builtins_property
 from collections.abc import Iterable, Iterator, Mapping
 from typing import Any, TypeVar, Union, overload
+from typing_extensions import Literal
 
 _T = TypeVar("_T")
 _S = TypeVar("_S", bound=type[Enum])
@@ -54,6 +55,7 @@ class EnumMeta(ABCMeta):
     @_builtins_property
     def __members__(self: type[_T]) -> types.MappingProxyType[str, _T]: ...
     def __len__(self) -> int: ...
+    def __bool__(self) -> Literal[True]: ...
     if sys.version_info >= (3, 11):
         # Simple value lookup
         @overload  # type: ignore[override]