BLD: remove outdated pin for `packaging` on macOS arm64

Why we don't need this anymore:

- This pin for `packaging` 20.5 was added in gh-18538.
- `setuptools` then upgraded its vendored `packaging` version to
  21.2 in its 59.1.0 release (see
  https://setuptools.pypa.io/en/latest/history.html#v59-1-0)
- And we then upgraded to `setuptools` 59.2.0 in commit 1adb70d99

We do not depend on `packaging` at all, it was a pin of a transitive
build dependency. So let's clean it up.

Tested locally as well with `python -m build --wheel` (which uses
`pyproject.toml` to build a wheel in an isolated venv), and then
checking the wheel tags and ensuring the wheel can be installed
with Pip.

[ci skip]

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