Example displays base map but not points

mapboxgl 0.10.2 in a Python 3.8.6 kernel in Jupyter:

Code

```python
with open('mapbox_token.txt','r') as f:
# Must be a public token, starting with `pk`
token = f.read().strip()

import os

import pandas as pd

from mapboxgl.utils import create_color_stops, df_to_geojson
from mapboxgl.viz import CircleViz

# Load data from sample csv
data_url = 'https://raw.githubusercontent.com/mapbox/mapboxgl-jupyter/master/examples/data/points.csv'
df = pd.read_csv(data_url)

# Create a geojson file export from a Pandas dataframe
df_to_geojson(df, filename='points.geojson',
properties=['Avg Medicare Payments', 'Avg Covered Charges', 'date'],
lat='lat', lon='lon', precision=3)

# Generate data breaks and color stops from colorBrewer
color_breaks = [0,10,100,1000,10000]
color_stops = create_color_stops(color_breaks, colors='YlGnBu')

# Create the viz from the dataframe
viz = CircleViz('points.geojson',
access_token=token,
height='400px',
color_property = "Avg Medicare Payments",
color_stops = color_stops,
center = (-95, 40),
zoom = 3,
below_layer = 'waterway-label'
)
viz.show()
```


I expected to see points on the map, like in the screenshot in the docs:

`df` looks vaguely reasonable / non-empty:

Also note the `UserWarning` mentioned in https://github.com/mapbox/mapboxgl-jupyter/issues/150#issuecomment-475405241, not sure if it's relevant:
```
/usr/local/lib/python3.8/site-packages/IPython/core/display.py:717: UserWarning: Consider using IPython.display.IFrame instead
warnings.warn("Consider using IPython.display.IFrame instead")
```

What am I missing? Thanks.

Start by running the supplied Python code in a Jupyter kernel and inspect the generated points.geojson. Trace how CircleViz consumes that file and renders points, using the mapboxgl.utils and mapboxgl.viz entry points shown in the example. Done means the example displays the point features as expected alongside the base map.

Written by the indexing model from the issue text.