Include cxx files in installed Python package (#204)

Maybe this is naive of me, but this should enable non-editable installs
to succeed as well?

Author: angus-g

pyproject.toml

@@ -45,6 +45,9 @@ Repository = "https://github.com/mesh-adaptation/animate"
 [tool.setuptools]
 packages = ["animate"]
 
+[tool.setuptools.package-data]
+animate = ["cxx/*.cxx"]
+
 [tool.ruff]
 line-length = 88