Cannot open file:// URLs on Android

Question

I'm trying to make my Python script open a local HTML file in the browser, or at least in web view, on Android (app Pydroid3).
Given that I specified the filename in filename:

# method 1
import webbrowser, os
webbrowser.open("file://"+os.path.realpath(filename))

This results in the message:
Cannot open file:// URLs on Android.

I am unable to find the first error as a sentence in a web search, is this error this uncommon?
When I un-quoted it I found only this other proposal:

# method 2
from urllib.request import pathname2url
url = 'file:{}'.format(pathname2url(os.path.abspath(filename)))
webbrowser.open(url)

This results in a toast stating:
Unable to start activity for android.intent.action.VIEW/file:/storage/emulated/0/folder/filename.html

How can I make it work?