fix obs_vector / var_vector return type

Author: ilia-kats

CHANGELOG.md

@@ -8,6 +8,13 @@ and this project adheres to [Semantic Versioning][].
 [keep a changelog]: https://keepachangelog.com/en/1.1.0/
 [semantic versioning]: https://semver.org/spec/v2.0.0.html
 
+## [0.3.8] (Unreleased)
+
+### Fixed
+
+- `obs_vector` and `var_vector` once again return Pandas Series objects when appropriate. This fixes
+  applying `sc.pl.umap` on a MuData object.
+
 ## [0.3.7]
 
 ### Fixed

src/mudata/_core/mudata.py

@@ -1247,7 +1247,7 @@ def _attr_vector(self, key: str, attr: str) -> np.ndarray:
                         f"There is no key {key} in MuData .{attr} but there is one in {m} .{attr}. Consider running `update_{attr}()` to update global .{attr}."
                     )
             raise KeyError(f"There is no key {key} in MuData .{attr} or in .{attr} of any modalities.")
-        return df[key].to_numpy()
+        return df[key].values
 
     @deprecated(Deprecation("0.3.4"))
     def obs_vector(self, key: str, layer: str | None = None) -> np.ndarray: