Fix import of deprecated gates from qiskit.extensions (#4366)

The monkeypatching PR didn't allow to import the deprecated gate names (such as Cu1Gate) to be imported from qiskit.extensions. This is fixed and a test added.

(cherry picked from commit 61b5e24)

qiskit/circuit/library/standard_gates/__init__.py

@@ -84,5 +84,17 @@
 from .y import YGate, CYGate
 from .z import ZGate, CZGate
 
-from .boolean_logical_gates import logical_and, logical_or
 from .multi_control_rotation_gates import mcrx, mcry, mcrz
+
+# deprecated gates
+from .boolean_logical_gates import logical_and, logical_or
+from .u1 import Cu1Gate
+from .u3 import Cu3Gate
+from .x import CnotGate, ToffoliGate
+from .swap import FredkinGate
+from .i import IdGate
+from .rx import CrxGate
+from .ry import CryGate
+from .rz import CrzGate
+from .y import CyGate
+from .z import CzGate

test/python/circuit/test_controlled_gate.py

@@ -904,6 +904,8 @@ def test_controlled_standard_gates(self, num_ctrl_qubits, gate_class):
 class TestDeprecatedGates(QiskitTestCase):
     """Test controlled of deprecated gates."""
 
+    import qiskit.extensions as ext
+
     import qiskit.extensions.standard.i as i
     import qiskit.extensions.standard.rx as rx
     import qiskit.extensions.standard.ry as ry
@@ -938,7 +940,18 @@ class TestDeprecatedGates(QiskitTestCase):
           (x.CXGate, x.CnotGate, []),
           (x.CCXGate, x.ToffoliGate, []),
           (y.CYGate, y.CyGate, []),
-          (z.CZGate, z.CzGate, []))
+          (z.CZGate, z.CzGate, []),
+          (i.IGate, ext.IdGate, []),
+          (rx.CRXGate, ext.CrxGate, [0.1]),
+          (ry.CRYGate, ext.CryGate, [0.1]),
+          (rz.CRZGate, ext.CrzGate, [0.1]),
+          (swap.CSwapGate, ext.FredkinGate, []),
+          (u1.CU1Gate, ext.Cu1Gate, [0.1]),
+          (u3.CU3Gate, ext.Cu3Gate, [0.1, 0.2, 0.3]),
+          (x.CXGate, ext.CnotGate, []),
+          (x.CCXGate, ext.ToffoliGate, []),
+          (y.CYGate, ext.CyGate, []),
+          (z.CZGate, ext.CzGate, []))
     @unpack
     def test_deprecated_gates(self, new, old, params):
         """Test types of the deprecated gate classes."""