Consider making None a normal Instance type with --strict-optional

[ddfisher]

NoneTyp is special cased in a lot of places, but—with strict optional—for the most part it behaves like a normal Instance type (and the special casing has to recreate that behavior). If we started tracking the truthiness of objects (see #1698), I think we could get rid of the vast majority of NoneTyp's special casing. (The only thing that I can think of that would remain its role in partial types.) This would make the code easier to reason about, and would have prevented some of the strict optional bugs we've seen so far.

[dmoisset]

My work on #1689 (see #1989) removed some references/special casing for NoneType inside find_isinstance_check. There's still another one there related to x is None and x is not None, probably those could be replaced by a more general analysis of expression1 is expression2 (if that is true, both expressions have the same type and some narrowings may be done, which can cover x is None as a special case).

[ilevkivskyi]

It looks like it is really not worth it at this stage. It will take a lot of work but benefits are questionable.