Change backup_parser to reflect shelve usage

tibi_hardlinks/backup_parser.py

@@ -29,11 +29,11 @@ def parse_property_text(text):
 
 
 def read_backup_properties(property_file: Path):
-    cached_result = tibi_cache.read(property_file.name)
-    if cached_result is None:
+    try:
+        data = tibi_cache[property_file.name]
+        return data
+    except KeyError:
         with open(property_file) as backup_properties:
             data = parse_property_text(backup_properties.read())
-        tibi_cache.write(property_file.name, data)
+        tibi_cache[property_file.name] = data
         return data
-    else:
-        return cached_result