Follow PEP 8 - Code lay-out - Imports

Trajectories/segments.py

@@ -11,12 +11,13 @@ segment is in ascending order.
 
 """
 
-import graph_tool
 import time
 import sys
 import pathlib
 from os import path
 
+import graph_tool
+
 if len(sys.argv) != 3:
     sys.exit("Required arguments: input-edgelist-file output-graph-tool-file")