Mark OSError.strerror as sometimes None

[brianschubert]

Refs #9864, follows from #12910

Like OSError.errno, strerror can sometimes be None. Usually, both attributes are None at the same time.

Since we added None to errno, it probably makes sense to do the same for strerror.

[github-actions]

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

[brianschubert]

I'm pleasantly surprised that there isn't a significant primer hit like there was with errno. Perhaps that's because strerror is often used in a string-formatting/printing context, where None would be legal anyway, e.g. raise MyCustomError(f"some message: {ex.strerror}").