cope with setuptools >= 61.0.0

[scgbckbone]

• context: [BUG] No longer possible to have multiple py files at top level pypa/setuptools#4013
• all python installation with >= 61.0.0 will create broken package after pip install -e . with error ModuleNotFoundError
• this change is backward-compatible and tested with older setuptools
• it would maybe be better to organize repo "correctly" as python package (i.e creating subdirectory ckbunker with __init__.py), then we can just list package=["ckbunker"] in setup.py