Make (cirq.X, cirq.Y, cirq.Z)**1 returned type as _Pauli{X,Y,Z} instead of {X,Y,Z}PowGate

[BichengYing]

Fix #4328

[MichaelBroughton]

In the __pow__ method there is a contract to return an instance of XPowGate. When returning self, you return a reference to an XPowGate and not a new object. That means with your code I could do:

assert cirq.X.global_shift == 0.0
a = [cirq.X**1.0, cirq.X**3.0]
a[0]._global_shift = 1.0
assert cirq.X.global_shift == 1.0
assert a[1].global_shift == 0.0

Which isn't great. I think modifying the code so that you are returning new instances in both cases would fix this.

[BichengYing]

In the __pow__ method there is a contract to return an instance of XPowGate. When returning self, you return a reference to an XPowGate and not a new object. That means with your code I could do:

assert cirq.X.global_shift == 0.0
a = [cirq.X**1.0, cirq.X**3.0]
a[0]._global_shift = 1.0
assert cirq.X.global_shift == 1.0
assert a[1].global_shift == 0.0

Which isn't great. I think modifying the code so that you are returning new instances in both cases would fix this.

Yeah, that is a good point. Totally agreed that _pow should return a new instance.

[MichaelBroughton]

LGTM