Pin rosdistro version and package the script to be usable without nix develop

Now the script works without updating rosdep, i.e., fetching the
database from the Internet.

pyproject.toml

@@ -0,0 +1,25 @@
+[project]
+name = "ros2nix"
+version = "0.0.1"
+authors = [
+  { name="Michal Sojka", email="wsh@2x.cz" },
+]
+description = "Convert ROS package.xml to Nix expression package.nix"
+readme = "README.md"
+requires-python = ">=3.8"
+classifiers = [
+    "Programming Language :: Python :: 3",
+    "License :: OSI Approved :: Apache 2",
+    "Operating System :: OS Independent",
+]
+
+[project.urls]
+Homepage = "https://github.com/wentasah/ros2nix"
+Issues = "https://github.com/wentasah/ros2nix/issues"
+
+[project.scripts]
+ros2nix = "ros2nix.ros2nix:main"
+
+[build-system]
+requires = ["setuptools>=61.0"]
+build-backend = "setuptools.build_meta"