Fix TypeError in issue #158/#159

[wcooley]

This makes only one more test pass (now only 3 failing) but at least prevents the TypeError when attempting to add a list and a tuple.

pytest output attached pytest-out-2.txt

[mahmoud]

It's taken a while to arrive at the correct course of action, but this pickle of an issue has finally been fixed in 19.0.0, just released. Thanks for your work, and your patience :)

[mahmoud]

Thought I was closing a different issue, reopening!

[allanlei]

@mahmoud @wcooley I've extended this PR in #298.