gpgme: fix cross, drop python2

This package very confusingly takes both python(2) and python3 as
arguments, and then mixes them up via an overlay in python-packages.
Since python2 is now marked INSECURE, let's just stick with python3.

Also fixes cross compilation.

pkgs/development/libraries/gpgme/default.nix

@@ -17,7 +17,6 @@
 , qtbase ? null
 , pythonSupport ? false
 , swig2 ? null
-, python ? null
 # only for passthru.tests
 , libsForQt5
 , python3
@@ -51,12 +50,16 @@ stdenv.mkDerivation rec {
     pkg-config
     texinfo
   ] ++ lib.optionals pythonSupport [
+    python3.pythonForBuild
     ncurses
-    python
     swig2
     which
   ];
 
+  buildInputs = lib.optionals pythonSupport [
+    python3
+  ];
+
   propagatedBuildInputs = [
     glib
     libassuan

pkgs/top-level/python-packages.nix

@@ -4131,7 +4131,7 @@ self: super: with self; {
 
   gpgme = toPythonModule (pkgs.gpgme.override {
     pythonSupport = true;
-    inherit python;
+    python3 = python;
   });
 
   gphoto2 = callPackage ../development/python-modules/gphoto2 { };