[Typing-sig] Supporting sys.platform.startswith in type checkers

Hello, I've been directed here from the typeshed community. Currently none of the type checkers and pyi linters support startswith with regards to sys.platform. It is however necessary for some more precise definitions for platform dependent constants - the sys.platform value for openbsd is openbsd{version} so comparison alone is not enough. Note that this is the recommended way to check for these other operating systems in the docs. Examples for such constants: mmap.ADV_* time.CLOCK_PROF time.CLOCK_UPTIME While obviously not a high priority issue, I'd still like to hear opinions. Thanks, Tal.