[harrison_creps] Use linear solver instead of matrix inverse

[UdohNakamura]

This patch replaces the explicit inverse computation la.inv(I - βQ) with la.solve(I - βQ, βQd) in price_single_beliefs for better numerical stability and efficiency.

The output remains identical to the previous implementation.

[UdohNakamura]

I also found that np.linalg.solve works the same here, so removing scipy might be possible.
However, I'm not confident about this, so I’d like to leave the decision to the reviewers.

[jstac]

Many thanks @UdohNakamura !

@mmcky This looks good to me. Please check if we can remove scipy.