minor and major reworks: rename swb to SRU, add a test for pdf parsing

major: rework mail to send mail as plaintext instead of html, preventing the bleed-in of html text

src/ui/dialogs/docuprint.py

@@ -2,7 +2,8 @@ from natsort import natsorted
 from PySide6 import QtWidgets
 
 from src import Icon
-from src.backend import Database, Semester
+from src.backend import Database
+from src.logic import Semester
 from src.utils.richtext import SemapSchilder, SemesterDocument
 
 from .dialog_sources.documentprint_ui import Ui_Dialog