python-chess last version renamed pop_count() to popcount()

[vitogit]

python-chess 0.17 changed pop_count() to popcount() so there is an error in the code when you run main.py https://github.com/niklasf/python-chess/blob/55c6031465d14f386f1f88754a9baafd1d8813db/CHANGELOG.rst

I don´t know if pip can install specific versions, maybe that can solve the problem (instlall the specific 0.15 version) and update the readme file with it. Or change the code to popcount()