add os.__all__

[tungol]

Another messy __all__. I tried to order the branches in a systematic way of increasing complexity: first version_info checks, then single platform and their versions, sorted alphabetically, then negative checks, then combinations, then or statements. No nesting the branches except where unavoidable (for complicated and/or conditions).

Side note, how are feeling about the precedent from #2286 to not use sys.platform checks on os.O_*? Some of them are certainly correct to be that way, but I feel like it's a little silly to not use the check when we can be confident about it.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Side note, how are feeling about the precedent from #2286 to not use sys.platform checks on os.O_*? Some of them are certainly correct to be that way, but I feel like it's a little silly to not use the check when we can be confident about it.

Platform checks are preferable, in my opinion. Back in 2018, we didn't have primer stubtest to verify our assumptions. Of course, some flags might still be awkward and their existence depends on the build and/or runtime environment, or the phase of the moon, but we should do what we can.