TYP: fix std/var output type for complex input - #31880

The np.std/np.var functions and ndarray.std/ndarray.var methods incorrectly returned complex dtypes for complex input. Because at runtime that would result in real output:

In [1]: c = np.array([1j, 1])

In [2]: np.std(c)
Out[2]: np.float64(0.7071067811865476)

In [3]: c.std()
Out[3]: np.float64(0.7071067811865476)

Miraculously, the fix didn't require adding new overloads this time. It even got rid of two!

Fully written by me (a human, ^{I think}); bug found by Claude Fable 5 (high) (here's the full audit for those interested).