Make ArchiveStats a dataclass

[btjanaka]

Description

The stats no longer need to be a namedtuple since we dropped Python 3.6 support. As we add more fields to the namedtuple, it will also get messy if there are users who still unpack it.

TODO

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go