==/!= doesn't work on optional type

[gvanrossum]

Behold:

def f(a: Optional[str], b: Optional[str]) -> bool:
  return a == b

We get this beautiful error with --strict-optional:

error: Unsupported left operand type for == (some union)

[rwbarton]

Maybe not obvious, but #1766 was a reduction of this issue.

[ddfisher]

No longer gives an error. I think this was fixed along with #1766 by #1836.