src/sage/matrix/matrix_double_dense.pyx: increase an FP tolerance

I recently hit,

  File "src/sage/matrix/matrix_double_dense.pyx", line 1629, in
  sage.matrix.matrix_double_dense.Matrix_double_dense.right_eigenvectors
  Failed example:
      spectrum[3]  # tol 1e-13
  Expected:
      (-1.0, [(1.0, -0.5, 2.0, 0.5)], 1)
  Got:
      (-1.0000000000000828,
       [(1.0, -0.4999999999999238, 1.9999999999997218, 0.4999999999999161)],
       1)
  Tolerance exceeded in 3 of 6:
      -0.5 vs -0.4999999999999238, tolerance 2e-13 > 1e-13
      2.0 vs 1.9999999999997218, tolerance 2e-13 > 1e-13
      0.5 vs 0.4999999999999161, tolerance 2e-13 > 1e-13

This commit changes that tolerance to 1e-12 and makes the failure go away.

src/sage/matrix/matrix_double_dense.pyx

@@ -1626,7 +1626,7 @@ cdef class Matrix_double_dense(Matrix_numpy_dense):
             (1.0, [(1.0, -0.666666666666633, 1.333333333333286, 0.33333333333331555)], 1)
             sage: spectrum[2]  # tol 1e-13
             (-2.0, [(1.0, -0.2, 1.0, 0.2)], 1)
-            sage: spectrum[3]  # tol 1e-13
+            sage: spectrum[3]  # tol 1e-12
             (-1.0, [(1.0, -0.5, 2.0, 0.5)], 1)
 
         A generalized eigenvalue problem::