¡Hola!

While rebuilding packages against python 3.15 we found that ttconv
produces some errors when used against it. The packages fails to build
from source [0].

Seems that the attribute _MISSING_TYPE from dataclasses is used and it
doesn't exists (at least) in python3.15. To fix it we should use
MISSING instead of _MISSING_TYPE.  I'm attaching a patch that fix this
issue.

[0] https://debusine.debian.net/debian/r-python-python3.15/artifact/4393731/