Use `astropy` provided by the distribution instead of installing it with pip

In Ubuntu, `rosdep` currently install `astropy` via `pip`, even though a [Debian package](https://launchpad.net/ubuntu/+source/astropy) is available in the official Ubuntu repositories. Unless there is a strong reason to install `astropy` system-wide with `pip`, which is generally discouraged and requires setting `PIP_BREAK_SYSTEM_PACKAGES=1`, could we switch to the `python3-astropy` package provided by `apt` instead? Or add a new entry for non-pip astropy? The PR #30575 where `astropy` was first introduced does not mention why pip was used instead of apt, even if the apt package [was already available at the time](https://launchpad.net/ubuntu/+source/astropy/4.0-4ubuntu1). References: - https://launchpad.net/ubuntu/+source/astropy - https://github.com/ros/rosdistro/blob/master/rosdep/python.yaml#L5017 - https://github.com/ros/rosdistro/pull/30575 - https://launchpad.net/ubuntu/+source/astropy/4.0-4ubuntu1 If you agree, I can open a PR.