Update native-messaging to use python3, clarify instructions

[dotproto]

Description

The following updates were made to the native-messaging example.

• Change the shebang in app/ping_pong.py explicitly use Python 3 instead of Python 2
• Change the app/ping_pong_win.bat runner to use Python 3 (untested)
• Updated the README
  • with more details on how to set up Python 3
  • explicitly tell the user to update files in app/ with the full paths of the files they reference
  • remove the Python 2 in the README as support for Python 2 ended in 2020
• Added a port.onDisconnect listener to add-on/background.js

Motivation

Update the example to function as expected.

Additional details

N/A

Related issues and pull requests

Fixes #560

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[dotproto]

@rebloor, at your suggestion I took a look at the native-messaging example and also wasn't able to get it to run locally on my test device using the instructions provided. Please take a look at the updated instructions and revise as appropriate.

[rebloor]

Thanks @dotproto I'll take a look first thing next week