Merge branch 'fix-189-key-error-pypdf' into 'master'

explicitly ask for keys

Closes #189

See merge request !253

src/strass/strass_app/utils.py

@@ -271,7 +271,7 @@ def validate_multiple_email(email):
 
 
 def _pdf_object_cleanup(pdf_file, obj):
-    for k in list(obj):
+    for k in list(obj.keys()):
         if k in [
             '/JavaScript',
             '/S',