generalize webcam detection with opt.source.isnumeric() (yolov3 #1462)

detect.py

@@ -21,7 +21,7 @@ from utils.torch_utils import select_device, load_classifier, time_synchronized
 def detect(save_img=False):
     out, source, weights, view_img, save_txt, imgsz = \
         opt.output, opt.source, opt.weights, opt.view_img, opt.save_txt, opt.img_size
-    webcam = source == '0' or source.startswith('rtsp') or source.startswith('http') or source.endswith('.txt')
+    webcam = source.isnumeric() or source.startswith('rtsp') or source.startswith('http') or source.endswith('.txt')
 
     # Initialize
     set_logging()