В свежем Сизифе падает при запуске:
> letters
Traceback (most recent call last):
  File "/bin/letters", line 45, in <module>
    from letters import main
  File "/usr/share/letters/letters/main.py", line 27, in <module>
    from .window import LettersWindow
  File "/usr/share/letters/letters/window.py", line 25, in <module>
    import pypandoc, weasyprint
ModuleNotFoundError: No module named 'weasyprint'

установка python3-module-weasyprint исправляет проблему.