New tests to validate type safety is preserved

[pipermerriam]

What was wrong?

From: https://github.com/ethereum/eth-utils/pull/151

Looks like flatten_return, reversed_return and sorted_return don't do a good job of retaining type safety. It's not easy to write a test for this now.

How can it be fixed?

Figure out how to easily write tests that demonstrate both positive and negative assertions about mypy checks.