BENCH: split spatial benchmark imports

As the different parts were added at different times, it's better to
import them separately so that only specific benchmarks stop working for
old scipy versions.

benchmarks/benchmarks/spatial.py

@@ -3,8 +3,22 @@
 import numpy as np
 
 try:
-    from scipy.spatial import (cKDTree, KDTree, SphericalVoronoi, distance,
-    ConvexHull, Voronoi)
+    from scipy.spatial import cKDTree, KDTree
+except ImportError:
+    pass
+
+try:
+    from scipy.spatial import distance
+except ImportError:
+    pass
+
+try:
+    from scipy.spatial import ConvexHull, Voronoi
+except ImportError:
+    pass
+
+try:
+    from scipy.spatial import SphericalVoronoi
 except ImportError:
     pass