Can't assign to () #5594

Mypy incorrectly complains about assignments to (). The simplest example of this is

() = []

This code executes without a problem, but mypy says error: can't assign to ()

A more real world use case might be code like this

(a, b), () = [[1, 2], []]

In this case the assignment to () is a concise way of asserting that the second list is empty.

• What is the actual behavior/output?
  error: can't assign to ()
• What is the behavior/output you expect?
  No type checking errors
• What are the versions of mypy and Python you are using?
  A build from the latest commit as of writing this, 614090b
• What are the mypy flags you are using?
  No flags

If this is a change you decide would be good then I believe it could be implemented by removing the the check on line 1930 of mypy/semanal.py and updating the tests.