matplotlib__matplotlib-25122

[Bug]: Windows correction is not correct in `mlab._spectral_helper`

Verdict: the fix is stated in the issue (Tier A).

The `np.abs` is not needed, and give wrong result for window with negative value, such as `flattop`.

The text states the fix: “The `np.abs` is not needed, and give wrong result for window with negative value, such as `flattop`.”