[python linting] enforce E731: Do not assign a lambda expression, use a def

Summary: Depends on D5202

Test Plan:
./test/functional/test_runner.py wallet_multiwallet
arc lint --everything

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5204