[lint] Fix variables shadowing python builtins in rpc_generateblock.py

Summary: Shadowing builtins can lead to unexpected behavior. Continuing work from D13740

Test Plan:

./test/functional/test_runner.py rpc_generateblock
flake8 rpc_generateblock.py | grep A001  # empty

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13741