Mark base node classes as ABCs

[bluetech]

The Node class hierarchy has some classes which are only meant to be used as base classes, never instantiated directly. These are Node, Collector, Item, File, FSCollector, PyCollector.

I propose we mark them as abstract i.e. inherit from abc.ABC. Reasons:

• Trying to instantiate fails quickly at runtime
• Type checkers issue an error if trying to instantiate
• The docs will show abstractmethod on the abstract methods that need to be implemented (collect or runtest), and show inheritance from ABC making it clear it's a base class

It seems somewhat hard to deprecate, and it will only cause errors when the code is already very wrong. So I propose to do it in pytest 8.0 without deprecation.