Mark SharedMemory.buf optional

[bersbersbers]

Close #14722

[github-actions]

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

[srittau]

I'm a bit wary of annotating this with | None unconditionally. In normal cases (i.e. before close() was called), buf cannot be None. It might be a better idea to annotate this with | MaybeNone, although the negative primer hits might indicate that this isn't a problem. I'm open to other opinions.

[bersbersbers]

I have no preference - I hadn't even heard of MaybeNone until now.

[srittau]

Ok, let's try None for now. We can always loosen it if it turns out to cause problems.