fix: pickle and deep copy classification models with EP

[olamarre]

• Removed unecessary code preventing pickling & copying
• Wrote a new test to check that pickling and copying works

Fixes #1105

Longer description:

• Removed all the "super" commands in the EPBase class since it does not have parent classes (raises an error otherwise - I suspect methods employing this command were never used or tested).
• Removed the __setstate__ and __getstate__ methods as they did not fully capture the state of an instance, making the implementation brittle. An error would be raised when pickling and unpickling an instance (for example, the self.always_reset attribute was missing). By removing these instances, Python now defaults to using the __dict__ method of the class, which works well and is much simpler.
• Added a test to validate that picking and deep copying now works with a classification model using expectation-propagation (ran test with pytest 8.3.4, GPy version 1.13.2 and Python 3.9.19)

[MartinBubel]

thanks @olamarre for your contribution. LGTM!
I have updated the changelog (noting your contribution) and I will merge it after the pipeline finishes.

If you need a new release featuring the fix, raise your hand - then I'll trigger the release pipeline.