Add Cython (#3217)

Cython required by pycocotools in certain cases, i.e. https://github.com/cocodataset/cocoapi/issues/172

requirements.txt

@@ -26,4 +26,5 @@ pandas
 
 # extras --------------------------------------
 thop  # FLOPS computation
+Cython  # for pycocotools https://github.com/cocodataset/cocoapi/issues/172
 pycocotools>=2.0  # COCO mAP