statistics: relax linear_regression input types

[hamdanal]

only len() and iteration are needed at runtime, this is accepted now

>>> import statistics
>>> d = {1: 10, 2: 20}
>>> statistics.linear_regression(d.keys(), d.values())
LinearRegression(slope=10.0, intercept=0.0)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[hauntsaninja]

Thanks!