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.
diff --git a/example/eigen.py b/example/eigen.py
index 8ab35ef..e69605d 100644
--- a/example/eigen.py
+++ b/example/eigen.py
@@ -13,6 +13,7 @@
 from example import double_mat_cm, double_mat_rm
 try:
     import numpy as np
+    import scipy
 except ImportError:
     # NumPy missing: skip test
     exit(99)