merge branch 'dt': move datetime utils into separate module

- use 'datetime.fromisoformat()' when possible (#7671)
- return a datetime-compatible object for invalid datetimes
  (instead of a 'str' value)

gallery_dl/extractor/schalenetwork.py

@@ -126,7 +126,7 @@ class SchalenetworkGalleryExtractor(SchalenetworkExtractor, GalleryExtractor):
         data = self.request_json(url, headers=headers)
 
         try:
-            data["date"] = text.parse_timestamp(data["created_at"] // 1000)
+            data["date"] = self.parse_timestamp(data["created_at"] // 1000)
             data["count"] = len(data["thumbnails"]["entries"])
             del data["thumbnails"]
         except Exception: