mock (not vcr) local test services

The response from this service should be mocked; otherwise, it becomes impossible for anyone else to reproduce without having the identical code running on their computer.
```python
@pytest.fixture
def client_proxy():
    yield Client(sandbox=True, proxy='http://127.0.0.1:3001')
```

This should use [`requests-mock`](https://pypi.org/project/requests-mock/)

/cc @felipaoo 