Follow-up: Ok, I've found out that the segfaults were caused by GTK+3 hack [1] which I've been testing yesterday and haven't turned off afterwards. Now keepnote and other Python apps run fine, so I'll continue investigating the mutex issue.
Don't ask me how a GTK+3 hack could possibly affect Python apps that are using GTK+2 - I have no idea :) [1] https://bugs.debian.org/744249#127
