fix: set dict correctly when unserializing (#1040)

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

src/boost_histogram/axis/__init__.py

@@ -96,7 +96,7 @@ def __setattr__(self, attr: str, value: Any) -> None:
 
     def __getattr__(self, attr: str) -> Any:
         if attr == "metadata":
-            return
+            return None
         raise AttributeError(
             f"object {self.__class__.__name__} has no attribute {attr}"
         )

src/boost_histogram/serialization/__init__.py

@@ -36,9 +36,9 @@ def from_uhi(data: dict[str, Any], /) -> histogram.Histogram:
     h = histogram.Histogram(
         *(_axis_from_dict(ax) for ax in data["axes"]),
         storage=_storage_from_dict(data["storage"]),
-        metadata=data.get("metadata"),
     )
     h[...] = _data_from_dict(data["storage"])
+    h.__dict__ = data.get("metadata", {})
     return h