Update relative ROOT logic (#4955)

* Update relative `ROOT` logic

* python 3.9 Path().is_relative_to() removal

models/tf.py

@@ -20,6 +20,7 @@ FILE = Path(__file__).resolve()
 ROOT = FILE.parents[1]  # YOLOv5 root directory
 if str(ROOT) not in sys.path:
     sys.path.append(str(ROOT))  # add ROOT to PATH
+ROOT = ROOT.relative_to(Path.cwd())  # relative
 
 import numpy as np
 import tensorflow as tf