Plenty of PyPI packages have external dependencies (e.g., anything that uses Cython will require a C compiler). SWIG isn't needed, since it is a source generato

Plenty of PyPI packages have external dependencies (e.g., anything that uses Cython will require a C compiler). SWIG isn't needed, since it is a source generator, and it's trivial to ship the generated source as part of the release process. GNAT is not commonly installed, but since it's part of GCC, it's packaged for every major OS and distribution. So there's one external dependency that is trivially installed, and many packages have similar requirements.