altair: Chart doesn't display, despite Mime representation enabled

I have an issue similar to #809 and the solution described didn’t work for me. Matplotlib can plot in Jupyter notebook. (I am using Jupyter notebook because Jupyter lab renders an empty page, or a page with only navigation headings, depending on the browser).

I use:

import altair as alt
from vega_datasets import data

alt.renderers.enable('notebook')
iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalLength',
    y='petalWidth',
    color='species'
)

and the output is empty: Out [1]:

Other tests that I ran are:

import sys
print(sys.version)
# 3.7.3 (default, Mar 27 2019, 09:23:15) 
# [Clang 10.0.1 (clang-1001.0.46.3)]
print(sys.executable)
# /usr/local/opt/python/bin/python3.7

class Test(object):
    def _repr_mimebundle_(self, include, exclude):
        return {'text/plain': "Mimebundle Repr"}
    def __repr__(self):
        return "Normal Repr"
    
Test()
# Mimebundle Repr

import IPython
IPython.__version__
# '7.5.0'