Use sys.platform instead of comments

[ymyzk]

Use of sys.platform is now recommended for platform-dependent APIs #2275. So, I update stubs which contains # Windows only or # Unix only to use if sys.platform ....

[ymyzk]

To fix failing tests https://travis-ci.org/python/typeshed/jobs/397420897, should we update tests in mypy, or should we avoid using sys.platform for some APIs to fix failing tests?

[gvanrossum]

Hm, I see the problem. Many of the O_* flags are present on some platforms and not others, with a finer granularity than sys.platform can indicate. So there's a convention to check for presence of a specific flag, which you see in the failing example code. I think for the O_ flags in particular it would be better to restore the original code (maybe add a clarifying comment as to why it's going against the recommendation).

[ymyzk]

I see the problem. I reverted stubs for O_* flags and add comments why we decided not to use sys.platform for now.

[JelleZijlstra]

I wonder if the same problem will happen with other flags changed in this PR. @gvanrossum perhaps you could check this PR against Dropbox's codebase to see if it introduces new mypy errors? (Our typed codebase doesn't tend to use low-level os features.)

[gvanrossum]

OK checking now...

[gvanrossum]

OK, it all works fine with the internal code bases.

[JelleZijlstra]

Thanks for checking!

[gvanrossum]

PS. In the future don't amend your commits during code review -- just push new commits, it's easier for the reviewers. They'll be squashed when we merge.

[ymyzk]

Sorry about that. I'll take care next time!