Pin Cython <3.1 in pyjnius build requirements

Add a build requirement for Cython <3.1 to the pyjnius recipe to prevent incompatibilities with newer Cython releases during the build. This updates recipes/pyjnius/meta.yaml to include the pinned Cython version while leaving existing host requirements and patches intact.

Author: FeodorFitsner

recipes/pyjnius/meta.yaml

@@ -6,5 +6,7 @@ patches:
   - mobile.patch
 
 requirements:
+  build:
+    - Cython <3.1
   host:
     - flet-libpyjni 1.0.1