PauliString repr does not preserve qubit order.

**Description of the issue**

If you create a PauliString 
```
p = cirq.PauliString({q2: cirq.Z, q1: cirq.Y, q0: cirq.X})
```
and round trip it via its repr, the qubit order does not match.
```
eval(repr(p)).qubits == p.qubits
>>> False
```

PauliString having ordered qubits was introduced in #4270
