Change keccak type return value to eth_typing.Hash32

[carver]

TODO: fill this stuff out...

What was wrong?

Code that produced the error

CODE_TO_REPRODUCE 

Full error output

ERROR_HERE 

Expected Result

This section may be deleted if the expectation is "don't crash".

EXPECTED_RESULT

Environment

# run this:
$ python -m eth_utils

# then copy the output here:
OUTPUT_HERE

How can it be fixed?

Fill this section in if you know how this could or should be fixed.