Add requirements.txt protobuf<=3.20.1 (#8017)

* Pin `protobuf==3.20.1`

* Update requirements.txt

* Update ci-testing.yml

requirements.txt

@@ -12,6 +12,7 @@ scipy>=1.4.1  # Google Colab version
 torch>=1.7.0
 torchvision>=0.8.1
 tqdm>=4.41.0
+protobuf<=3.20.1  # https://github.com/ultralytics/yolov5/issues/8012
 
 # Logging -------------------------------------
 tensorboard>=2.4.1