deve/py-xmltodict: fix build with py36 by always open file as utf-8 encoded

This fix the 'UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 2587: ordinal not in range(128)'

Obtained from: https://github.com/martinblech/xmltodict/pull/246
Approved by: portmgr blanket (trivial build fix)