Fix shadowing of bin Python keyword

Summary: bin is a Python built-in: https://docs.python.org/3/library/functions.html#bin

Test Plan:
Run any build:
./build-configurations.py build-without-wallet

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7717