How to set the basemap layers of a WebMap using the ArcGIS Python API?

[JonathanBailey]

Using the ArcGIS Python API, I've create a WebMap and two Layers (one containing features, the other containing labels):

cbmt = WebMap()
cbmt_geometry_layer = Layer(url = <url of geometry layer>)
cbmt_text_layer = Layer(url = <url of text layer>)‍‍‍‍‍‍‍‍‍

I'd like to add both of these layers to the map as basemap layers, like you can in ArcGIS Online or Portal when adding layers to a web map. The WebMap.basemap property is read-only. Is there a property on the WebMap.add_layer() method that specifies to add layers as basemap layers?

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