fixed Item instantiation error in Pytest 8

[str0zzapreti]

• Changed finalizer to use a dummy Class to avoid ABC instantiation error
• Pytest made this change in version 8.0 to enforce standards for their base classes
• Added tests to validate fixture handling at class and module scope
• I find the chance of a name collision highly unlikely, but if requested I will add a note to the documentation instructing users to avoid naming any of their test classes 'Fakeboi' while using pytest-retry. I hope common sense will prevail.

Thanks to @adamtheturtle for highlighting this change and pushing an initial fix!

[str0zzapreti]

@adamtheturtle As far as I can tell this fix is functionally identical to the one you proposed in https://github.com/str0zzapreti/pytest-retry/pull/34. I experimented with a couple of other Item subclasses including Function and FunctionDefinition but settled on Class as the simplest way to resolve the error while ensuring the necessary fixture scopes are all handled correctly. I prefer this over using Module because it minimizes imports, but otherwise your fix is just as good (ignoring the added tests and comment updates).