Topological sort of a graph, allowing better console (and other) outputs.

Uses algorithm borrowed from networkx graph library to sort a graph in
topological order. The method is only used by output plugins, as
internal plumbery does not really care about the node order.

Also includes a bonobo.util.python.require function that helps importing
thing in a package-less context, or when there are conflict with site
package names.

tests/structs/test_graphs.py

@@ -1,5 +1,7 @@
 import pytest
 
+from unittest.mock import sentinel
+
 from bonobo.constants import BEGIN
 from bonobo.structs import Graph
 
@@ -41,3 +43,30 @@ def test_graph_add_chain():
     g.add_chain(identity, identity, identity)
     assert len(g.nodes) == 3
     assert len(g.outputs_of(BEGIN)) == 1
+
+
+def test_graph_topological_sort():
+    g = Graph()
+
+    g.add_chain(
+        sentinel.a1,
+        sentinel.a2,
+        sentinel.a3,
+        _input=None,
+        _output=None,
+    )
+
+    assert g.topologically_sorted_indexes == (0, 1, 2)
+    assert g[0] == sentinel.a1
+    assert g[1] == sentinel.a2
+    assert g[2] == sentinel.a3
+
+    g.add_chain(
+        sentinel.b1,
+        sentinel.b2,
+        _output=sentinel.a2,
+    )
+
+    assert g.topologically_sorted_indexes == (0, 3, 4, 1, 2)
+    assert g[3] == sentinel.b1
+    assert g[4] == sentinel.b2