Pin python-rapidjson<=1.20 for py3.8 wheel support

tritonclient pulls in python-rapidjson; 1.21+ ships no Python 3.8
wheels, so pip builds from source and fails on the slim image (no g++).
1.20 is the last release with cp38 manylinux wheels.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: chrisdengso

requirements.txt

@@ -19,6 +19,8 @@ onnxruntime==1.16.3
 PyTurboJPEG==1.7.5
 exifread
 tritonclient[http,grpc]
+# transitive dep of tritonclient; >=1.21 has no py3.8 wheels (source build needs g++)
+python-rapidjson<=1.20
 imageio
 aiohttp
 tenacity