Add packaging as a required dependency

[ruffsl]

This add a missing declared dependency for the packaging library, as a follow up to:

• Fix ament_black for new get_sources API #13

As can be seen in use here:

ament_black/ament_black/ament_black/main.py

Line 36 in 81af884

from packaging.version import Version

As for the minimum version number, I simply opted to use whatever shipped with Ubuntu 20.04.

• https://packages.ubuntu.com/focal/python3-packaging

We could probable go further back, as it looks to have been around awhile:

• https://github.com/pypa/packaging/blame/c385b58c4d0e4d66f5c8273ebb155801c92aadf3/src/packaging/version.py#L161

[nachovizzo]

Thanks @ruffsl!
I will bake a release tomorrow