irc: add safe_text_length and use abc.ABC for AbstractBot

[Exirel]

Description

Tin. I was always a bit annoyed by the say method that does too much on its own, and I know that @half-duplex wanted a way to know the safe length for an IRC message without having to reinvent the wheel each time. I hope this will help, even if just a little.

Working on this part, I saw that hostmask is used but not defined by the abstract class, so I took this opportunity to use abc.ABC and the abc.abstractmethod decorator.

By doing so, I also replaced the KeyError when the hostmask is not defined to return None instead. This is a very minor breaking change. I'm not sure anyone is using bot.hostmask, and they probably don't use it when the bot is not connected/hasn't join a channel yet.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

I can think of no objections. It's mostly my code, just moved somewhere else. 😁

[Exirel]

Your code, with more tests. 😁