Allow Ellipsis as annotation for inferred type

PEP 484 says that if an annotation is missing then the type assumed to be Any

def f(x) -> None:
    reveal_type(x) # Revealed type is 'Any'
def g(x: int):
    pass
reveal_type(g(1)) # Revealed type is 'Any'

However, it is not clear how to say to type checker that it should infer a missing type, rather than assume that it is Any. It was proposed by @ncoghlan to use Ellipsis for this purpose:

def f(x: ...) -> None:
    ...
def g(x: int) -> ...:
    ...

I am opening this issue, so that this idea will not be forgotten.

先阅读 PEP 484 和此 issue 中的示例；没有确定任何实现文件或测试。在定义验收测试之前，先明确预期的 Ellipsis 语义以及受影响的类型检查器行为，因为该 issue 记录的是一项提案，而不是范围明确的变更。