Plotly Express density_mapbox, attribute 'opacity' raises ValueError

[tvaucher]

Description

Hello,
When passing the opacity attribute different than None to px.density_mapbox, it raises a ValueError.

Expected output

Taking the example in the doc and wanting to set the opacity to 0.5:

Minimal reproducible code

import pandas as pd
df = pd.read_csv('https://raw.githubusercontent.com/plotly/datasets/master/earthquakes-23k.csv')

import plotly.express as px
fig = px.density_mapbox(df, lat='Latitude', lon='Longitude', z='Magnitude', radius=10,
                        center=dict(lat=0, lon=180), zoom=0,
                        mapbox_style="stamen-terrain", opacity=0.5)
fig.show()

Current workaround

It's possible to change the opacity by not adding the attribute in the constructor but adding it after using fig.update_traces(opacity=0.5). But I'll open a PR with a fix so you don't have to use that.