Add implicit iteration to CouplingMap (#9051)

* Add implicit iteration to CouplingMap

This adds an `__iter__` method that passes through to iteration through
the underlying rustworkx `EdgeList` (after construction).  This commit
does not add the other methods needed to make `CouplingMap` into a
Python sequence, such as `__getitem__` and `__len__`, because
constructing the `EdgeList` has a nonzero cost (the rustworkx
implementation eagerly collects the edge indices into a `Vec` to iterate
over), and could lead to hidden performance pitfalls.  The edges also
have no particular order associated with them, so implementing
`Sequence` would be a misnomer.

As a side-effect, this makes it legal to pass a `CouplingMap` instance
to the `CouplingMap` constructor, but this is not a true copy
constructor (which in Python are perhaps better spelled as
`CouplingMap.copy()` so immutable objects can avoid the new-object
allocation cost without messing around with `__new__`), since fields
like `description` would not be copied.  This commit takes no position
on whether a true copy constructor is a good idea.

* Fix terrible manual sorting

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/transpiler/coupling.py

@@ -79,6 +79,9 @@ class CouplingMap:
         """
         return self.graph.edge_list()
 
+    def __iter__(self):
+        return iter(self.graph.edge_list())
+
     def add_physical_qubit(self, physical_qubit):
         """Add a physical qubit to the coupling graph as a node.
 

releasenotes/notes/iterable-couplingmap-b8f0cbb1b34a2005.yaml

@@ -0,0 +1,15 @@
+---
+features:
+  - |
+    :class:`.CouplingMap` is now implicitly iterable, with the iteration being
+    identical to iterating through the output of :meth:`.CouplingMap.get_edges()`.
+    In other words,
+
+    .. code-block:: python
+
+      from qiskit.transpiler import CouplingMap
+      coupling = CouplingMap.from_line(3)
+      list(coupling) == list(coupling.get_edges())
+
+    will now function as expected, as will other iterations.  This is purely a
+    syntactic convenience.

test/python/transpiler/test_coupling.py

@@ -442,6 +442,12 @@ class CouplingTest(QiskitTestCase):
         expected = [(0, 1), (1, 2), (2, 3)]
         self.assertEqual(expected, edge_list, f"{edge_list} does not match {expected}")
 
+    def test_implements_iter(self):
+        """Test that the object is implicitly iterable."""
+        coupling = CouplingMap.from_line(3)
+        expected = [(0, 1), (1, 0), (1, 2), (2, 1)]
+        self.assertEqual(sorted(coupling), expected)
+
 
 class CouplingVisualizationTest(QiskitVisualizationTestCase):
     @unittest.skipUnless(optionals.HAS_GRAPHVIZ, "Graphviz not installed")