Advance to scipy >=1.5.0

[cwhanse]

#1827 adds iam.convert and iam.fit. These functions need a bounded, vectorized minimization. That is provided in scipy>=1.5.0 via method='power' method for optimize.minimize. scipy is on 1.11.x now, 1.5.0 was release 20/6/2020.

Describe the solution you'd like
Advance minimum scipy to 1.5.0

Describe alternatives you've considered
Test imported version of scipy and throw error (in iam.convert and iam.fit) if scipy<1.5.0