Nix: Allow to build against current unstable

For Subversion we have to start using the attribute "dev" for the include path.
16.09 supports this as well and for 16.03 a fallback with a TODO note is
included.

pkgs/python-packages-overrides.nix

@@ -1,56 +1,57 b''
 # Overrides for the generated python-packages.nix
 #
 # This function is intended to be used as an extension to the generated file
 # python-packages.nix. The main objective is to add needed dependencies of C
 # libraries and tweak the build instructions where needed.
 
 { pkgs, basePythonPackages }:
 
 let
   sed = "sed -i";
 in
 
 self: super: {
 
   subvertpy = super.subvertpy.override (attrs: {
-    SVN_PREFIX = "${pkgs.subversion}";
+    # TODO: johbo: Remove the "or" once we drop 16.03 support
+    SVN_PREFIX = "${pkgs.subversion.dev or pkgs.subversion}";
     propagatedBuildInputs = attrs.propagatedBuildInputs ++ [
       pkgs.aprutil
       pkgs.subversion
     ];
     preBuild = pkgs.lib.optionalString pkgs.stdenv.isDarwin ''
       ${sed} -e "s/'gcc'/'clang'/" setup.py
     '';
   });
 
   mercurial = super.mercurial.override (attrs: {
     propagatedBuildInputs = attrs.propagatedBuildInputs ++ [
       self.python.modules.curses
     ] ++ pkgs.lib.optional pkgs.stdenv.isDarwin
       pkgs.darwin.apple_sdk.frameworks.ApplicationServices;
   });
 
   pyramid = super.pyramid.override (attrs: {
     postFixup = ''
       wrapPythonPrograms
       # TODO: johbo: "wrapPython" adds this magic line which
       # confuses pserve.
       ${sed} '/import sys; sys.argv/d' $out/bin/.pserve-wrapped
     '';
   });
 
   Pyro4 = super.Pyro4.override (attrs: {
     # TODO: Was not able to generate this version, needs further
     # investigation.
     name = "Pyro4-4.35";
     src = pkgs.fetchurl {
       url = "https://pypi.python.org/packages/source/P/Pyro4/Pyro4-4.35.src.tar.gz";
       md5 = "cbe6cb855f086a0f092ca075005855f3";
     };
   });
 
   # Avoid that setuptools is replaced, this leads to trouble
   # with buildPythonPackage.
   setuptools = basePythonPackages.setuptools;
 
 }