Motivation for this change

This changes 2 small parts about buildPythonPackage. I'm happy to split them into two PRs if you prefer, but I figured I'd combine them since they require a rebuild of all python packages.

1. send pip install output to stderr. The nix integration in direnv (https://direnv.net/, from @zimbatm) relies on nix-shell --run CMD producing only the output from CMD on stdout (it dumps the environment from inside nix-shell and recreates it in your current shell). The python shellHook logs the following pip boilerplate to stdout, which breaks this functionality:

Obtaining file:///home/tim/dev/python/test
Installing collected packages: test
  Running setup.py develop for test
Successfully installed test

This is clearly output that belongs on stderr, so I've added a >&2 redirect in the shell hook.

2. allow passing developmentPrefix to buildPythonPackage. This is used only in the shellHook, and allows a specific path to be used for development installation, rather than creating a tempfile each time. This keeps the $PATH reproducible, which is important if you store it somewhere to be reused at a later time (since nix-shell can take a while on complex inputs, I cache the results used by direnv to set my environment). The default behaviour is unchanged.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.