Context manager open(file) as f fixes (#7289)

* Flask context manager `open()` fix

* Additional read context manager fixes

utils/flask_rest_api/example_request.py

@@ -1,12 +1,18 @@
-"""Perform test request"""
+# YOLOv5 🚀 by Ultralytics, GPL-3.0 license
+"""
+Perform test request
+"""
+
 import pprint
 
 import requests
 
 DETECTION_URL = "http://localhost:5000/v1/object-detection/yolov5s"
-TEST_IMAGE = "zidane.jpg"
+IMAGE = "zidane.jpg"
 
-image_data = open(TEST_IMAGE, "rb").read()
+# Read image
+with open(IMAGE, "rb") as f:
+    image_data = f.read()
 
 response = requests.post(DETECTION_URL, files={"image": image_data}).json()