No line breaks for long lines with unpacking

[tweakimp]

Operating system: Win10 64bit
Python version: 3.3
Black version: 19.3b0
Does also happen on master: yes
See black playground

When there are many long names to the right side of an = sign, black stacks them vertically.
However, for many long names on the left side there is no solution and the line stays too long.

I dont know if this is intended or if there is no other way to solve this, but maybe you have a solution for me :)

[JelleZijlstra]

This is a bug, but it's already being tracked at #445. Thanks for your report!