Mark OSError.strerror as sometimes None - #12974

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

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}").