Adding platform information to stdlib/VERSIONS

It might be a good idea to add platform availability information to stdlib/VERSIONS, instead of using the sys.platform hack we're currently using in the source files. This has the same rationale for introducing VERSIONS in the first place:

• Remove platform checks and the required indentation from stub files.
• Enables clearer type checker messages (e.g. "Module does not exist on this platform", instead of "Item X doesn't exist in module Y")
• Generally an unambiguous way of indicating that a module doesn't exist.
• Edit: Collect all information about module availability in one place.

One way to do this is an HTTP/E-Mail header-like key/value syntax:

abc: 3.0-; platforms=linux,darwin
def: 3.7-3.10; platforms=!windows,!linux
ghi: 3.9-

This would allow future extension when type checkers are equipped to ignore unknown keys:

abc: 3.0-; header1=foo; header2=bar

Do we need a way do indicate that a certain platform only supports a certain module starting from a certain Linux version?

从 stdlib/VERSIONS 以及当前使用 sys.platform 检查的源文件开始。检查其中如何表示模块可用性，以及类型检查器如何使用这些信息。定义平台键/值格式，包括特定版本的平台支持和未知键的处理方式；当提案的规格足够明确、可以进行实现时，即视为完成。