Cap rpy2 version (#3261)

*Issue #, if available:* Recent releases appear to have broken some
functionality
https://github.com/awslabs/gluonts/actions/runs/15924891079/job/44919811812

*Description of changes:*


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dev setup

Author: lostella

requirements/requirements-extras-r.txt

@@ -1 +1 @@
-rpy2~=3.5
+rpy2~=3.5,<3.6