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?

Start with stdlib/VERSIONS and the source files that currently use sys.platform checks. Review how module availability is represented there and how type checkers consume it. Define the platform key/value format, including version-specific platform support and handling of unknown keys; done means the proposal is specified well enough for implementation.