automate update from py25 to py26 for dependencies?

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

I tried to install pidgin-otr which pulls in chat/pidgin/buildlink3.mk 
which includes chat/libpurple/buildlink3.mk which includes 
sysutils/py-dbus/buildlink3.mk (by default).

I have installed:
pidgin-2.6.2nb2 
libpurple-2.6.2nb1
py25-dbus-0.83.0

Now PYTHON_VERSION_DEFAULT is 26. So building pidgin-otr fails with:

  ===> Creating toolchain wrappers for pidgin-otr-3.2.0nb4
  ERROR: py26-dbus>=0.23.4 is not installed; can't buildlink files.

How can I get pkgsrc to attempt to build py26-dbus automatically?

I also tried to workaround with (in pidgin-otr):

     make clean
     make PKG_OPTIONS.libpurple=-dbus

But no change.