Motivation for this change

...fix build by moving swig2 to nativeBuildInputs

Looking at the future of this package, the original source appears to be unmaintained, and the pypi entry's homepage is an old broken googlecode.com address.

It's probably worth looking at switching to one of the forks, but which one I'm not particularly sure. I'm not sure how one would gauge the credibility of these sources - https://github.com/pybox2d/pybox2d probably looks the most promising, but then again, anyone can register the "pybox2d" github organization. The bonus from switching upstream would be being able to pull the complete package from github including the tests, which we could then enable.

Edit: Looking at this closer, I realized I was able to remove the pkgs-box2d buildInput entirely. This dependency turns out to be completely unnecessary - the pypi package includes its own bundled copy of the source (I know - ew.) and building without pkgs-box2d results in a binary-identical .so.

This has the side-effect of enabling this package on darwin, which was previously restricted by the apparently linux-only pkgs-box2d.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.