DeprecationWarning for call to pkg_resources.declare_namespace() on import.

setuptools 67.3.0 has deprecated `pkg_resources.declare_namespace()`: https://setuptools.pypa.io/en/latest/history.html#v67-3-0

This is being triggered by importing `intuit-oauth`:

```
DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('intuitlib')`.
Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
declare_namespace(pkg)
```

To remain a namespace package, the `__init__.py` may be deleted. But I'm not sure the package needs to be a namespace package, and we may be able to use an empty `__init__.py` instead.

Start by inspecting how the package defines the intuitlib namespace and how importing intuit-oauth triggers setuptools' pkg_resources.declare_namespace() warning. Determine whether intuitlib should remain a namespace package or use an empty __init__.py, then verify that importing the package no longer emits the deprecation warning.

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