Fix reference to deprecated SingleQubitGate (#788)

- This is breaking one of the openfermion notebooks.

src/openfermion/circuits/primitives/ffft.py

@@ -83,7 +83,7 @@ def _circuit_diagram_info_(self, args: cirq.CircuitDiagramInfoArgs
         return cirq.CircuitDiagramInfo(wire_symbols=symbols)
 
 
-class _TwiddleGate(cirq.SingleQubitGate):
+class _TwiddleGate(cirq.Gate):
     r"""Gate that introduces arbitrary FFT twiddle factors.
 
     Realizes unitary gate $\omega^{k\dagger}_n$ that phases creation
@@ -108,6 +108,9 @@ def __init__(self, k, n):
         self.k = k
         self.n = n
 
+    def _num_qubits_(self) -> int:
+        return 1
+
     def _circuit_diagram_info_(self, args: cirq.CircuitDiagramInfoArgs
                               ) -> cirq.CircuitDiagramInfo:
         if args.use_unicode_characters: