Why is it even encoding at all?  Almost every other part of Python 3 forces you to be explicit about bytes/string conversion.  For example:

struct.pack("10s", x.encode('utf-8'))

Given that automatic conversion is documented, it's not clear what can be done at this point.  However, there are very few other parts of Python 3 that perform implicit string-byte conversions like this (at least that I know of off-hand).