Powers of symbolic sums inconsistence

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```
sage: (-3*x-3)^2
9*(x + 1)^2

but

sage: (-I*x-I)^2
(-I*x - I)^2
sage: (4*x + 12)^(3/2)
(4*x + 12)^(3/2)
```
Expected would be `-(x + 1)^2` and `8*(x + 3)^(3/2)`.

The `power::eval` code in Pynac is being cleaned up atm.

Component: **symbolics**

Reviewer: **Ralf Stephan**

_Issue created by migration from https://trac.sagemath.org/ticket/24768_

