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

pyproject.toml

@@ -12,7 +12,7 @@ license = { file = "LICENSE" }
 dependencies = []
 
 [project.optional-dependencies]
-dev = ["pytest>=8"]
+dev = ["pytest>=9,<10", "ruff>=0.16,<0.17", "mypy>=2.4,<3"]
 
 [tool.setuptools.packages.find]
 where = ["src"]
@@ -20,3 +20,16 @@ where = ["src"]
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 pythonpath = ["src"]
+
+[tool.ruff]
+line-length = 88
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "UP", "FLY", "B"]
+
+[tool.ruff.lint.per-file-ignores]
+"src/higher_lower/game_data.py" = ["E501"] # assignment data kept verbatim
+
+[tool.mypy]
+strict = true
+mypy_path = "src"