Make it easier to (de)serialize RunReport

By making it, and the classes it depends upon, into attrs classes rather
than dataclass. This means we can use cattrs to structure() and
unstructure() it to/from a dict, and then to/from JSON, so we can store
it more easily.

Author: Hamish Downer

src/sortition_algorithms/utils.py

@@ -7,9 +7,9 @@
 import sys
 from abc import ABC, abstractmethod
 from collections.abc import Generator, Iterable, Mapping, Sequence
-from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any
 
+from attrs import define, field
 from tabulate import tabulate
 
 from sortition_algorithms import errors
@@ -82,30 +82,30 @@ class ReportLevel(enum.Enum):
     CRITICAL = 2
 
 
-@dataclass
+@define(slots=True)
 class RunLineLevel:
     line: str
     level: ReportLevel
     log_level: int = logging.NOTSET
 
 
-@dataclass
+@define(slots=True)
 class RunTable:
     headers: list[str]
     data: list[list[str | int | float]]
 
 
-@dataclass
+@define(slots=True)
 class RunError:
     error: Exception
     is_fatal: bool
 
 
+@define
 class RunReport:
     """A class to hold a report to show to the user at the end"""
 
-    def __init__(self) -> None:
-        self._data: list[RunLineLevel | RunTable | RunError] = []
+    _data: list[RunLineLevel | RunTable | RunError] = field(factory=list)
 
     def __bool__(self) -> bool:
         """