Does three things:

1. adds ChatMemberUpdated.difference as suggested by @harshil21 - the naming of the method is up for discussion, if you have other suggestions :)
2. Adds new constants to tg.constants/Update which can be used to set allowed_updates more conveniently
3. adds an example on how to use ChatMemberHandler and above stuff

pre-commit/pylint is failing locally for some reason. let's see how CI behaves.

Checklist for PRs

• Added .. versionadded:: version, .. versionchanged:: version or .. deprecated:: version to the docstrings for user facing changes (for methods/class descriptions, arguments and attributes)
• Created new or adapted existing unit tests
• Added myself alphabetically to AUTHORS.rst (optional)

If the PR contains API changes (otherwise, you can delete this passage)

• If relevant:

  • Added new constants at telegram.constants and shortcuts to them as class variables
  • Added new handlers for new update types
  • Added new filters for new message (sub)types
  • Added or updated documentation for the changed class(es) and/or method(s)
  • Updated the Bot API version number in all places: README.rst and README_RAW.rst (including the badge), as well as telegram.constants.BOT_API_VERSION