update python code, remove __future__ imports, remove six, use absl-py for flags, update examples

ortools/graph/samples/simple_max_flow_program.py

@@ -13,7 +13,6 @@
 # [START program]
 """From Taha 'Introduction to Operations Research', example 6.4-2."""
 # [START import]
-from __future__ import print_function
 from ortools.graph import pywrapgraph
 # [END import]
 

ortools/graph/samples/simple_min_cost_flow_program.py

@@ -13,7 +13,6 @@
 # [START program]
 """From Bradley, H. and M., 'Applied Mathematical Programming', figure 8.1."""
 # [START import]
-from __future__ import print_function
 from ortools.graph import pywrapgraph
 # [END import]