Trac #27615: py3: use of "round" in matrix_double_dense.pyx

Use `N(...)` instead of `round(...)` in a doctest in
matrix_double_dense.pyx, so py3 doctests pass.

URL: https://trac.sagemath.org/27615
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Frédéric Chapoton

src/sage/matrix/matrix_double_dense.pyx

@@ -2078,7 +2078,7 @@ cdef class Matrix_double_dense(Matrix_dense):
             sage: S.round(4)
             [ 9.508    0.0    0.0]
             [   0.0 0.7729    0.0]
-            sage: [round(sqrt(abs(x)),4) for x in (S*S.transpose()).eigenvalues()]
+            sage: [N(sqrt(abs(x)), digits=4) for x in (S*S.transpose()).eigenvalues()]
             [9.508, 0.7729]
 
         U and V are orthogonal matrices::