Add pyproject.toml for the Day 21 repository

Adopted from the base (020-snake-game): requires-python >=3.13, no runtime dependencies, a dev extra with pytest, ruff and mypy. Only the description (day 21) and the repository URL differ from the base.

Task: MIL-001#1
Closes #1

pyproject.toml

@@ -0,0 +1,54 @@
+[build-system]
+requires = ["setuptools>=77"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "snake-game"
+version = "0.1.0"
+description = "Snake game from Udemy's 100 Days of Code (day 21), built object-oriented with Python turtle graphics: food, score and game over."
+readme = "README.md"
+requires-python = ">=3.13"
+license = "AGPL-3.0-only"
+license-files = ["LICENSE"]
+authors = [{ name = "Jens Tirsvad Nielsen" }]
+keywords = ["snake", "game", "turtle", "udemy", "100-days-of-code"]
+classifiers = [
+    "Programming Language :: Python :: 3 :: Only",
+    "Programming Language :: Python :: 3.13",
+    "Topic :: Education",
+    "Topic :: Games/Entertainment",
+]
+# The game runs on the standard library only (turtle needs tkinter).
+dependencies = []
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=8",
+    "ruff>=0.6",
+    "mypy>=1.11",
+]
+
+[project.urls]
+Repository = "https://git.tirsystem.com/Tirsvad-Udemy-100-days-of-code/021-snake-game"
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src", "tests"]
+
+[tool.ruff]
+line-length = 88
+target-version = "py313"
+src = ["src", "tests"]
+extend-exclude = ["framework", "build"]
+
+[tool.ruff.lint]
+select = ["E", "F", "W", "I", "N", "UP", "B", "SIM"]
+
+[tool.mypy]
+python_version = "3.13"
+strict = true
+files = ["src", "tests"]
+mypy_path = ["src", "tests"]