contibution-stats: fix utf-8 output
Output utf-8 correctly to avoid errors such as this:
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 4: ordinal not in range(128)
(we're an international team with fancy names and characters).
I think myjosserand already filled a MR to fix the same issue by calling the python3 interpreter instead. If that one is merged, this one won't be necessary.