Fix!: Allow init to be walked to track its dependencies

[themisvaltinos]

Fixes an issue in python model's serialization where class dependencies referenced inside __init__ were not added to the serialised env, for example:

class ReferencedClass:
    def __init__(self, value: int):
        self.value = value

    def get_value(self) -> int:
        return self.value

class ClassThatReferencesAnother:
    def __init__(self, x: int):
        self.helper = ReferencedClass(x * 2)

This is because in metaprogramming the build_env() function was skipping all dunder methods including __init__ so the dependencies were never added to the serialized environment.

[georgesittas]

There may be an edge case where this is breaking, FYI. If __init__ references a global variable, for example, which was previously treated as metadata-only because it appeared elsewhere, now it will be picked up and should not be treated as metadata-only.

Not sure if we can do much about this, just mentioning so that we can defensively mark the PR accordingly.

[georgesittas]

The Windows CI job is failing:

FAILED tests/core/test_context.py::test_requirements - AssertionError: assert {'ipywidgets'...test_package'} == {'ipywidgets'...test_package'}
  
  Extra items in the left set:
  'pendulum'
  
  Full diff:
    {
        'ipywidgets',
        'numpy',
        'pandas',
  +     'pendulum',
        'test_package',
    }
==== 1 failed, 2244 passed, 3 skipped, 7239 warnings in 336.78s (0:05:36) =====

Should we update the test?