Fix Python 3.8 compatibility

There are some differences in how the inspect stdlib module behaves
between python 3.8 and newer versions of python. This was causing
divergence in the test and qpy behavior where inspect was used to
determine different aspects of a gate (either whether label was
supported as an arg or find the number of free parameters). This commit
fixes this by adjusting the code to handle both newer versions of
inspect as well as older ones.

qiskit/qpy/binary_io/circuits.py

@@ -28,6 +28,7 @@
 from qiskit.circuit import library, controlflow, CircuitInstruction
 from qiskit.circuit.classicalregister import ClassicalRegister, Clbit
 from qiskit.circuit.gate import Gate
+from qiskit.circuit.singleton_gate import SingletonGate
 from qiskit.circuit.controlledgate import ControlledGate
 from qiskit.circuit.instruction import Instruction
 from qiskit.circuit.quantumcircuit import QuantumCircuit
@@ -292,9 +293,14 @@ def _read_instruction(file_obj, circuit, registers, custom_operations, version,
             if "label" in inspect.signature(gate_class).parameters:
                 gate = gate_class(*params, label=label)
             else:
-                gate = gate_class(*params)
                 if label is not None:
-                    gate.label = label
+                    if issubclass(gate_class, SingletonGate):
+                        gate = gate_class(*params, label=label)
+                    else:
+                        gate = gate_class(*params)
+                        gate.label = label
+                else:
+                    gate = gate_class(*params)
         if condition_tuple:
             gate = gate.c_if(*condition_tuple)
     if circuit is None:

test/python/circuit/gate_utils.py

@@ -25,7 +25,7 @@ def _get_free_params(fun, ignore=None):
     Returns:
         list[str]: The name of the free parameters not listed in ``ignore``.
     """
-    ignore = ignore or []
+    ignore = ignore or ["kwargs"]
     free_params = []
     for name, param in signature(fun).parameters.items():
         if param.default == Parameter.empty and param.kind != Parameter.VAR_POSITIONAL: