pwntools: use system python 2

Because we will remove the python@2 formula end of 2019.

Closes #47873.

Signed-off-by: Michka Popoff <michkapopoff@gmail.com>

Formula/pwntools.rb

@@ -5,7 +5,7 @@ class Pwntools < Formula
   homepage "https://github.com/Gallopsled/pwntools"
   url "https://github.com/Gallopsled/pwntools/archive/3.12.2.tar.gz"
   sha256 "8e048b514ee449b4c76f4eba1b4fcd48fdefd1bf04ae4c62b44e984923d2e979"
-  revision 1
+  revision 2
 
   bottle do
     cellar :any
@@ -16,7 +16,8 @@ class Pwntools < Formula
   end
 
   depends_on "openssl@1.1"
-  depends_on "python@2" # does not support Python 3
+  # Has a 4.0 beta release with Python 3 support
+  uses_from_macos "python@2"
 
   conflicts_with "moreutils", :because => "Both install `errno` binaries"