Fix coreml quantization (WongKinYiu#434)

Instead of importing platform, you can check it by `sys.platform`. By the way, `prefix` is a missing value in this script.

export.py

@@ -89,12 +89,12 @@
         ct_model = ct.convert(ts, inputs=[ct.ImageType('image', shape=img.shape, scale=1 / 255.0, bias=[0, 0, 0])])
         bits, mode = (8, 'kmeans_lut') if opt.int8 else (16, 'linear') if opt.fp16 else (32, None)
         if bits < 32:
-            if platform.system() == 'Darwin':  # quantization only supported on macOS
+            if sys.platform == 'Darwin':  # quantization only supported on macOS
                 with warnings.catch_warnings():
                     warnings.filterwarnings("ignore", category=DeprecationWarning)  # suppress numpy==1.20 float warning
                     ct_model = ct.models.neural_network.quantization_utils.quantize_weights(ct_model, bits, mode)
             else:
-                print(f'{prefix} quantization only supported on macOS, skipping...')
+                print('quantization only supported on macOS, skipping...')
 
         f = opt.weights.replace('.pt', '.mlmodel')  # filename
         ct_model.save(f)