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Ignoring whether it's a good idea, there are a few mistakes in the implementation:

- [`__getattr__`](https://docs.python.org/3/reference/datamodel.html#object.__getattr__) is called "when the default attribute access **fails**". To control _all_ attribute access, you want [`__getattribute__`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__).
  - By the same token, `super().__getattr__` is not defined.
- [`__delete__`](https://docs.python.org/3/reference/datamodel.html#object.__delete__) is for descriptors only; that should be [`__delattr__`](https://docs.python.org/3/reference/datamodel.html#object.__delattr__).
- Deletion methods (`__delattr__` and `__delete__`) don't return anything.

And some surface-level things could be improved:

- Pylint: "Unnecessary `else` after `return`; remove the `else` and de-indent the code inside it ([R1705:no-else-return](https://pylint.readthedocs.io/en/latest/user_guide/messages/refactor/no-else-return.html))". In effect, convert your if-else's to guard statements.
- Pylint: "Consider explicitly re-raising using `raise AttributeError(error) from error` ([W0707:raise-missing-from](https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/raise-missing-from.html))". This changes part of the error message from "During handling of the above exception, another exception occurred:" to "The above exception was the direct cause of the following exception:"
- `assert` is a statement, not a function, so the parentheses are not needed and can be confusing
- Methods should be separated by 1 newline, not 2, [per PEP 8](https://peps.python.org/pep-0008/#blank-lines)
- Add a `__repr__`! It's easy and helps a lot with debugging.

```
class DictProxy:
    # [docstring snipped for brevity]

    def __init__(self, dictionary):
        """
        Initialize the DictProxy with a dictionary.
        """
        self._dictionary = dictionary

    def __setattr__(self, name, value):
        """
        Redirect non-underscore attribute assignments to dictionary updates.
        """
        if name[0] == "_":
            super().__setattr__(name, value)
        else:
            self._dictionary[name] = value

    def __getattribute__(self, name):
        """
        Redirect non-underscore attribute reads to dictionary reads.
        """
        if name[0] == "_":
            return super().__getattribute__(name)

        try:
            return self._dictionary[name]
        except KeyError as error:
            # hasattr fails if the exception isn't an AttributeError
            raise AttributeError(error) from error

    def __delattr__(self, name):
        """
        Redirect non-underscore attribute deletes to dictionary deletes.
        """
        if name[0] == "_":
            super().__delattr__(name)
        else:
            del self._dictionary[name]

    def __repr__(self):
        """Show construction."""
        clsname = type(self).__name__
        return f'{clsname}({self._dictionary!r})'


def test():
    dictionary = {"a": 1, "b": 2, "c": 3}
    proxy = DictProxy(dictionary)

    assert hasattr(proxy, "a")
    assert hasattr(proxy, "b")
    assert hasattr(proxy, "c")
    assert not hasattr(proxy, "d")

    # [etc.]
```