Enforce type formatter in JSON ITL extra keys during export

The patch implemented in (!10 (merged) for %Version 1.1.1) didn't work as expected because every extra parameters were formatted as strings but pointing_designer should be formatted as a boolean.

fmt_json(
    EventWindow(
        'foo',
        t_start='2032-09-23T05:15:45',
        t_end='2032-09-23T05:26:15',
        pointing=None,  # null value
        pointing_designer=False,  # boolean value
        pointing_description='bar',  # str value
        baz='qux',  # other parameter (not extra)
    )
)

should report now:

{
    'name': 'foo',
    'start_time': '2032-09-23T05:15:45.000Z',
    'end_time': '2032-09-23T05:26:15.000Z',
    'pointing': None,
    'pointing_designer': False,
    'pointing_description': 'bar',
    'type': 'OBSERVATION',  # appended
    'parameters': {'baz': 'qux'},
}

Other fix included in this merge request:

• jupyter-book dependency in RTD config (closes #16 (closed))