[sffbase] Make convert_hex_to_string() compatible with both Python 2 and Python 3 (sonic-net#165)

When called with Python 3, `str.strip(binascii.unhexlify(ret_str))` will raise an exception: ` descriptor 'strip' requires a 'str' object but received a 'bytes'`. This patch (`binascii.unhexlify(ret_str).decode("utf-8").strip()`) will work properly with both Python 2 and Python 3.

Fixes sonic-net/sonic-platform-common#160

Author: jleveque

sonic_platform_base/sonic_sfp/sffbase.py

@@ -35,7 +35,7 @@ def convert_hex_to_string(self, arr, start, end):
             ret_str = ''
             for n in range(start, end):
                 ret_str += arr[n]
-            return str.strip(binascii.unhexlify(ret_str))
+            return binascii.unhexlify(ret_str).decode("utf-8").strip()
         except Exception as err:
             return str(err)