scons: Switch to Python 3

Reasons:

Python 2.7 will EOL very soon [0]:
DEPRECATION: Python 2.7 will reach the end of its life on January 1st,
2020. Please upgrade your Python as Python 2.7 won't be maintained after
that date. A future version of pip will drop support for Python 2.7.
More details about Python 2 support in pip, can be found at
https://pip.pypa.io/en/latest/development/release-process/#python-2-support

SCons 4.0.0 will drop Python 2.7 Support [1]:
https://raw.githubusercontent.com/SConsProject/scons/rel_3.1.2/src/CHANGES.txt

[0]: From the SCons build output previous to this commit (i.e. with Python 2.7).
[1]: https://raw.githubusercontent.com/SConsProject/scons/rel_3.1.2/src/CHANGES.txt

pkgs/development/tools/build-managers/scons/common.nix

@@ -1,19 +1,16 @@
 { version, sha256 }:
 
-{ stdenv, fetchurl, python2Packages }:
+{ stdenv, fetchurl, python3Packages }:
 
-let name = "scons";
+python3Packages.buildPythonApplication rec {
-in python2Packages.buildPythonApplication {
+  pname = "scons";
-  name = "${name}-${version}";
+  inherit version;
 
   src = fetchurl {
-    url = "mirror://sourceforge/scons/${name}-${version}.tar.gz";
+    url = "mirror://sourceforge/scons/${pname}-${version}.tar.gz";
     inherit sha256;
   };
 
-  # Fix a regression in 3.0.0 (causes build errors for some packages)
-  patches = stdenv.lib.optional (version == "3.0.0") ./print-statements.patch;
-
   setupHook = ./setup-hook.sh;
 
   meta = with stdenv.lib; {

pkgs/development/tools/build-managers/scons/print-statements.patch

@@ -1,13 +0,0 @@
-diff --git a/src/engine/SCons/Script/SConscript.py b/src/engine/SCons/Script/SConscript.py
-index 558e28f9..8fea9c4d 100644
---- src/engine/SCons/Script/SConscript.py
-+++ src/engine/SCons/Script/SConscript.py
-@@ -5,8 +5,6 @@
- 
- """
- 
--from __future__ import print_function
--
- #
- # __COPYRIGHT__
- #