alot: add patch for email composition

This version is not usable without this patch.
See pazz/alot#1468.

(cherry picked from commit b6d4530)

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

@@ -1,4 +1,4 @@
-{ stdenv, lib, buildPythonPackage, python, fetchFromGitHub, isPy3k
+{ stdenv, lib, buildPythonPackage, python, fetchFromGitHub, fetchpatch, isPy3k
 , notmuch, urwid, urwidtrees, twisted, python_magic, configobj, mock, file, gpgme
 , service-identity, glibcLocales
 , gnupg ? null, sphinx, awk ? null, procps ? null, future ? null
@@ -19,6 +19,15 @@ buildPythonPackage rec {
     sha256 = "0wmhb23fgbn2x15llfwzkf392h4a32f7j1fb7gc4w95wr8jhwk2r";
   };
 
+  patches = [
+    # can't compose email if signature is set: https://github.com/pazz/alot/issues/1468
+    (fetchpatch {
+      name = "envelope-body.patch";
+      url = "https://github.com/pazz/alot/commit/28a4296c7f556c251d71d9502681980d46d9fa55.patch";
+      sha256 = "1iwvmjyz4mh1g08vr85ywhah2xarcqg8dazagygk19icgsn45w06";
+    })
+  ];
+
   nativeBuildInputs = lib.optional withManpage sphinx;
 
   propagatedBuildInputs = [