Add missing scipy run-time dep to eigen test

scipy is imported in pybind11/eigen.h when it encounters a sparse
matrix, which gets tested in the eigen test.

example/eigen.py

@@ -13,6 +13,7 @@ from example import double_row, double_col
 from example import double_mat_cm, double_mat_rm
 try:
     import numpy as np
+    import scipy
 except ImportError:
     # NumPy missing: skip test
     exit(99)