mypy does not recognize keyword arguments to json_serializable_dataclass instances

[kevinsung]

To reproduce:

1. Create a file test.py in cirq/ with the following contents:

import dataclasses
from cirq import protocols


@dataclasses.dataclass
class A:
    a: int


@protocols.json_serializable_dataclass
class B:
    b: int


A(a=0)
B(b=0)

2. Execute check/mypy. This gives the following output:

cirq/test.py:16: error: Unexpected keyword argument "b" for "B"

[mpharrigan]

looks fairly hopeless: https://mypy.readthedocs.io/en/stable/additional_features.html#caveats-known-issues

[dabacon]

Do we want to remove json_serializable_dataclass decorator? It doesn't look like ti is used anywhere.

[kevinsung]

It's used once in experiments/cross_entropy_benchmarking.py but that shouldn't prevent us from removing it.

[mpharrigan]

I would not object to removing it, if the drawbacks exceed the marginal benefit

[mpharrigan]

this annotation should be deprecated: #4460

use the dataclass helper function in a regular ol' method instead

[95-martin-orion]

Resolved by #5208.