Adds filters.IS_BOT, a ready to use filter that matches updates whose telegram.Update.effective_user is a bot. Until now nothing in the filters module exposed telegram.User.is_bot, so telling bot authored messages apart from human ones had to happen inside the callback. With this filter the decision can be made at handler registration time, and ~filters.IS_BOT covers the human only case.

The filter is implemented as an UpdateFilter over Update.effective_user, which is the same approach the existing PREMIUM_USER and USER_ATTACHMENT filters take for other telegram.User attributes. It returns False when the update carries no user at all, for example for channel posts.

The docstring also points out that in groups a bot only receives messages from other bots when privacy mode is disabled, since that is the setting that decides whether the filter can ever match in a multi bot chat.

Fixes #5315.

Check-list for PRs

• Added .. versionadded:: NEXT.VERSION, .. versionchanged:: NEXT.VERSION, .. deprecated:: NEXT.VERSION or ``.. versionremoved:: NEXT.VERSION` to the docstrings for user facing changes (for methods/class descriptions, arguments and attributes)
• Created new or adapted existing unit tests
• Documented code changes according to the CSI standard
• Added myself alphabetically to AUTHORS.rst (optional)
• Added new classes & modules to the docs and all suitable __all__ s
• Checked the Stability Policy in case of deprecations or changes to documented behavior