Fix lint, format and typing findings and pin dev tools

Order imports, format with ruff, make the account key constants Final
so mypy --strict accepts them, and type the test helpers. Configure
ruff and mypy in pyproject.toml and pin the dev tool versions. All
modules, classes and functions already carry Doxygen comments;
doxygen Doxyfile builds without warnings.

Task: MIL-004#2
Closes #18

src/higher_lower/game_logic.py

@@ -1,5 +1,5 @@
 """@file game_logic.py
-@brief Pure functions of the game: drawing accounts, describing them and checking guesses.
+@brief Pure game functions: draw accounts, describe them, check guesses.
 """
 
 import random
@@ -9,9 +9,9 @@ from higher_lower.constants import (
     ACCOUNT_TEMPLATE,
     CHOICE_A,
     CHOICE_B,
+    KEY_COUNTRY,
     KEY_DESCRIPTION,
     KEY_NAME,
-    KEY_COUNTRY,
 )
 from higher_lower.game_data import Account, data