stats: Ignore invalid attributes

This makes it easier to debug invalid stats usage.

weblate/utils/stats.py

@@ -180,7 +180,9 @@ def has_review(self):
     def cache_key(self):
         return f"stats-{self._object.cache_key}"
 
-    def __getattr__(self, name):
+    def __getattr__(self, name: str):
+        if name.startswith("_"):
+            raise AttributeError
         if self._data is None:
             self._data = self.load()
         if name.endswith("_percent"):