BUG: fix `gaussian_kernel_estimate` on S390X
BUG: fix `gaussian_kernel_estimate` on S390X

Reference issue

https://github.com/scipy/scipy/issues/13512

What does this implement/fix?

used `math.power()` over exponentiation operator to fix unexpected behavior on S390X.

Additional information

https://github.com/scipy/scipy/issues/13512#issuecomment-780246692

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