Trivial dataclass cleanups: (GH-6218)

Trivial dataclass cleanups: (GH-6218)

- When adding a single element to a list, use .append() instead of
  += and creating a new list.
- For consistency, import the copy module, instead of just deepcopy. This
  leaves only a module at the class level, instead of a function.
- Improve some comments.
- Improve some whitespace.
- Use tuples instead of lists.
- Simplify a test.
(cherry picked from commit f96ddade0094d162cb6c2fd7255c5e8a90b5c37d)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>