make TopoOptions JSON serializable

[mattijn]

Currently when doing:

import geojson
import topojson
import altair as alt

feature_1 = geojson.Feature(
    geometry=geojson.Polygon([[[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]]),
    properties={"name":"abc"}
)
feature_2 = geojson.Feature(
    geometry=geojson.Polygon([[[1, 0], [2, 0], [2, 1], [1, 1], [1, 0]]]),
    properties={"name":"def"}
)
data = geojson.FeatureCollection([feature_1, feature_2])

tj = topojson.Topology(data, prequantize=False).to_dict()

# inline topojson data object
data_topojson = alt.InlineData(values=tj, format=alt.DataFormat(feature='data',type='topojson')) 

# chart object
alt.Chart(data_topojson).mark_geoshape(
).encode(
    color="properties.name:N"
).properties(
    projection={'type': 'identity', 'reflectY': True}
)

Results in:

TypeError: Object of type 'TopoOptions' is not JSON serializablec:\programdata\miniconda3\lib\json\encoder.py in default(self, o)
    178         """
    179         raise TypeError("Object of type '%s' is not JSON serializable" %
--> 180                         o.__class__.__name__)
    181 
    182     def encode(self, o):

TypeError: Object of type 'TopoOptions' is not JSON serializable

This is because tj['options'] is a class and not a dict.