Bump narwhals from 2.20.0 to 2.21.0 (#38)

Author: dependabot[bot]

requirements.txt

@@ -19,7 +19,7 @@ kiwisolver==1.5.0
 MarkupSafe==3.0.3
 matplotlib==3.10.9
 microSWIFTtelemetry==0.3.7
-narwhals==2.20.0
+narwhals==2.21.0
 nest-asyncio==1.6.0
 numpy==2.4.4
 packaging==26.2