Added warning message about Mbed OS Py3 compatability

Author: cmonr

mbed/mbed.py

@@ -2987,6 +2987,10 @@ def main():
         log(ver+"\n")
         sys.exit(0)
 
+    # Python 3 backwards compatability warning
+    if sys.version_info[0] == 3:
+        warning("Usage of Python 3 requires Mbed OS >= v5.9")
+
     pargs, remainder = parser.parse_known_args()
     status = 1