Use tobytes instead of tostring in netutil.py

Summary: tostring was deprecated in pything 3.7 and this is spewing errors

Test Plan:

./test/functional/test_runner.py rpcbind_test.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2211