support for pygtk 3+

[arcturusannamalai]

minor changes will enable support for pygtk+3

[Nanolx]

I know this is ancient, but I forked undobuffer before it was available on github and my version does support Gtk+ 3, feel free to use it or to backport it upstream:

https://gitlab.com/Nanolx/bashstyle-ng/-/blob/master/ui/undobuffer.py

or

https://github.com/Nanolx/bashstyle-ng/blob/master/ui/undobuffer.py

depending on whether I'll still use it when switching to Gtk+ 4 some time, I may update it further...