Replace use of numpy.lib.scimath with preferred numpy.emath (#10892)

`numpy.emath` is the preferred alias to `numpy.lib.scimath`, and direct
access to the latter is scheduled for removal in Numpy 2.

(cherry picked from commit af24227)

Author: jakelishman

qiskit/quantum_info/synthesis/qsd.py

@@ -175,7 +175,7 @@ def _demultiplex(um0, um1, opt_a1=False, opt_a2=False, *, _depth=0):
     else:
         evals, vmat = scipy.linalg.schur(um0um1, output="complex")
         eigvals = evals.diagonal()
-    dvals = np.lib.scimath.sqrt(eigvals)
+    dvals = np.emath.sqrt(eigvals)
     dmat = np.diag(dvals)
     wmat = dmat @ vmat.T.conjugate() @ um1