My Python build doesn't ship with anything GUI-toolkit related (and isn't part of the JailBreak itself, but of a dedicated package).
There's a fairly ancient GTK stack available on the device itself, but I refuse to lower myself to building against such ancient code ;p. baf & twobob have shown that it was useable, though
.
Other than that, for UI stuff, you used to have the Java Kindlet stuff, but that's apparently been deprecated, and has fairly particular and annoying limitations, so let us speak of it no more.
That leaves playing with the framebuffer manually, which is what KOReader does, for instance.
(There also used to be a Qt build laying around, but I'm not sure it ever was completely usable on FW 5.x).