Add Cython (#3217)

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

Add Cython (#3217)
7b863a34 · Glenn Jocher · GitHub · 7b0eb952 · 7b863a34
--- a/requirements.txt
+++ b/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