If you run that through c++filt, it says it's "typeinfo for Fl_Widget". Typeinfo means RTTI, which is a c++ feature FLTK does not use, and so disabled in our build, as it saves some cpu and library size.
So depending on how pyfltk works, if it needs RTTI itself you need a custom FLTK build; if it does not, then you just need to add -fno-rtti to pyfltk's build flags.