Fix unused imports in contrib/ python scripts

Summary: Preparation before including a linter for this.

Test Plan:

./contrib/testgen/gen_base58_test_vectors.py valid 5
./contrib/testgen/gen_base58_test_vectors.py invalid 5

In both case there should be no python error

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2445