Mark classes as not instantiable

From python/typeshed#14558: Some classes (like re.Pattern) are not instantiable at runtime:

>>> import re
>>> re.Pattern()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create 're.Pattern' instances

Currently, there is no working way to mark classes as such. Ideas to mark classes:

• Annotate the return type of __new__ as NoReturn/Never.
• Set __new__ to None (similar to how __hash__ works).
• A new decorator.

从 issue 中的运行时示例和链接的 typeshed 参考 python/typeshed#14558 开始。将提议的 new 注解、将 new 设置为 None 以及装饰器与现有的 typing 行为进行比较。完成的标准是，项目已经就如何表达一个类无法被实例化达成一致，并定义了预期的语义。