advanced SimpleNamespace typing

the same way we now have TypedDict, it would be nice to have typed SimpleNamespace with equivalent semantics, so that nested structures can be easily checked. eg

class MySubType(TypedSimpleNamespace):
   bar: int

class MyType(TypedSimpleNamespace):
   foo: MySubType

# correct
a: MyType = SimpleNamespace(foo=SimpleNamespace(bar=1))
# incorrect, `bar` should be a number
a: MyType = SimpleNamespace(foo=SimpleNamespace(bar='abc'))

该 issue 没有提供 repository 文件或测试入口；首先将所请求的嵌套 TypedSimpleNamespace 语义与 PEP 589 TypedDict 进行比较。在查找实现和测试之前，先明确所示有效示例和无效示例的类型检查行为及范围。