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/art.py

@@ -9,23 +9,37 @@ string, so Doxygen does not read the backticks in the art as markup.
 ## The game logo.
 logo = (
     "\n"
-    r"    __  ___       __             " "\n"
-    r"   / / / (_)___ _/ /_  ___  _____" "\n"
-    r"  / /_/ / / __ `/ __ \/ _ \/ ___/" "\n"
-    r" / __  / / /_/ / / / /  __/ /    " "\n"
-    r"/_/ ///_/\__, /_/ /_/\___/_/     " "\n"
-    r"   / /  /____/_      _____  _____" "\n"
-    r"  / /   / __ \ | /| / / _ \/ ___/" "\n"
-    r" / /___/ /_/ / |/ |/ /  __/ /    " "\n"
-    r"/_____/\____/|__/|__/\___/_/     " "\n"
+    r"    __  ___       __             "
+    "\n"
+    r"   / / / (_)___ _/ /_  ___  _____"
+    "\n"
+    r"  / /_/ / / __ `/ __ \/ _ \/ ___/"
+    "\n"
+    r" / __  / / /_/ / / / /  __/ /    "
+    "\n"
+    r"/_/ ///_/\__, /_/ /_/\___/_/     "
+    "\n"
+    r"   / /  /____/_      _____  _____"
+    "\n"
+    r"  / /   / __ \ | /| / / _ \/ ___/"
+    "\n"
+    r" / /___/ /_/ / |/ |/ /  __/ /    "
+    "\n"
+    r"/_____/\____/|__/|__/\___/_/     "
+    "\n"
 )
 
 ## The "vs" shown between the two accounts.
 vs = (
     "\n"
-    r" _    __    " "\n"
-    r"| |  / /____" "\n"
-    r"| | / / ___/" "\n"
-    r"| |/ (__  ) " "\n"
-    r"|___/____(_)" "\n"
+    r" _    __    "
+    "\n"
+    r"| |  / /____"
+    "\n"
+    r"| | / / ___/"
+    "\n"
+    r"| |/ (__  ) "
+    "\n"
+    r"|___/____(_)"
+    "\n"
 )

src/higher_lower/constants.py

@@ -2,11 +2,13 @@
 @brief Constants shared by the game modules.
 """
 
+from typing import Final
+
 ## Keys of an account dictionary.
-KEY_NAME = "name"
-KEY_FOLLOWER_COUNT = "follower_count"
-KEY_DESCRIPTION = "description"
-KEY_COUNTRY = "country"
+KEY_NAME: Final = "name"
+KEY_FOLLOWER_COUNT: Final = "follower_count"
+KEY_DESCRIPTION: Final = "description"
+KEY_COUNTRY: Final = "country"
 
 ## The two choices the player can type, in lower case.
 CHOICE_A = "a"

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