Spec: Version and platform checking underspecified

Currently, the "Version and platform checking" section of the typing spec is very barebones. It basically says:

Type checkers are expected to understand simple version and platform checks. [...] Don’t expect a checker to understand obfuscations like "".join(reversed(sys.platform)) == "xunil".

Of course, there's a world of difference between these two forms.

The type stubs document goes into a bit more of detail, and reflects the currently implemented functionality quite well, as far as I know. It would be a good starting point.

Maybe it would also make sense to support platform checks using startswith(), as that is officially recommended in the Python docs, and is required to properly support some Unix variants, like FreeBSD, that include the version number in the platform string. See also python/typeshed#11876.

首先，将 typing 规范中的“Version and platform checking”部分与链接的类型存根文档进行比较，后者反映了当前行为。查看报告中链接的 Python 文档和 typeshed issue，然后定义受支持的检查，并决定 startswith() 是否属于规范。完成的标准是：规范准确描述预期的版本和平台检查行为。