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/backend/__init__.py

@@ -1,6 +1,5 @@
 __all__ = [
     "AdminCommands",
-    "Semester",
     "AutoAdder",
     "AvailChecker",
     "BookGrabber",
@@ -9,16 +8,15 @@ __all__ = [
     "NewEditionCheckerThread",
     "recreateElsaFile",
     "recreateFile",
-    "Catalogue"
+    "Catalogue",
 ]
 
 from .admin_console import AdminCommands
+from .catalogue import Catalogue
 from .create_file import recreateElsaFile, recreateFile
 from .database import Database
 from .documentation_thread import DocumentationThread
-from .semester import Semester
 from .thread_bookgrabber import BookGrabber
 from .thread_neweditions import NewEditionCheckerThread
 from .threads_autoadder import AutoAdder
 from .threads_availchecker import AvailChecker
-from .catalogue import Catalogue