python.pkgs.ipython: allow prompt_toolkit 2.0

This applies the appropriate upstream patch:
8e256bd373

pkgs/development/python-modules/ipython/default.nix

@@ -3,6 +3,7 @@
 , buildPythonPackage
 , fetchPypi
 , pythonOlder
+, fetchpatch
 # Build dependencies
 , glibcLocales
 # Test dependencies
@@ -34,6 +35,12 @@ buildPythonPackage rec {
     substituteInPlace setup.py --replace "'gnureadline'" " "
   '';
 
+  # Upgrade to prompt_toolkit 2.0
+  patches = fetchpatch {
+    url = https://github.com/ipython/ipython/commit/8e256bd37373f98580ba1ef1d3fcfd7976802238.patch;
+    sha256 = "1d9qy2z21n4frf15g4aj7xi011d1d3qc31gs27f2v23j0gv69r9h";
+  };
+
   buildInputs = [ glibcLocales ];
 
   checkInputs = [ nose pygments ];