[y_cable] Skip ImportError in y_cable.py (sonic-net#154)

When build python3 xcvrd, it tries to do basic check which will import this y_cable.py. However, not all platform supports python3 API now, so it could cause an issue when importing sonic_platform.platform

We skip the ImportError here to make the builder happy. And this is safe because:
- If any python package is not available, there will be exception when use it
- Vendors know their platform API version, they are responsible to use correct python version when importing this file.

sonic_y_cable/y_cable.py

@@ -6,12 +6,16 @@
 
 try:
     import struct
-
-    import sonic_platform.platform
     from sonic_py_common import logger
-
+    import sonic_platform.platform
 except ImportError as e:
-    raise ImportError(str(e) + " - required module not found")
+    # When build python3 xcvrd, it tries to do basic check which will import this file. However,
+    # not all platform supports python3 API now, so it could cause an issue when importing 
+    # sonic_platform.platform. We skip the ImportError here. This is safe because:
+    #   1. If any python package is not available, there will be exception when use it
+    #   2. Vendors know their platform API version, they are responsible to use correct python
+    #   version when importing this file.
+    pass
 
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