DEPS: clarifying setuptools's status as a dependency 
In `install.rst`, we list `setuptools` as a runtime dependency. If that is the case, we don't need to vendor `packaging` #41199 since it's vendored by `pkg_resources`, which is part of `setuptools`. And we need to update `setup.cfg` and `README.md` to reflect this.

If `setuptools` is not a runtime dependency, vendoring `packaging` is necessary. And, we need to list it as a test and optional dependency because we are using `pkg_resources` in `pandas/plotting/_core.py` and `pandas/tests/plotting/test_backend.py` #41503. An alternative of `pkg_resources` for accessing entry points is `importlib.metadata`, which is only available after Python 3.8. And `importlib.metadata` is the suggested way to do this according to https://setuptools.readthedocs.io/en/latest/pkg_resources.html.