Fix the mininode/messages imports in example.py

Summary:
When splitting messages.py from mininode.py, a wildcard import from
messages has been kept in mininode to avoid breaking the tests. This
diffs update exemple.py so it does not depend on this workaround.

Test Plan:

./test/functional/test_runner.py example_test

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2599