REL-935: Release v 8.0.0

Updates to version 8.0.0.

The other changes are:

• Loading the docs version specifier from pypoetry.toml so we don't have to set it in a separate place.
• Reverting typing.Union[] that was a red herring for fixing autodoc problems
• Removing astropy from autodoc_mock_imports that was the actual problem with autodoc.
• Fixing some docstring indentation.