python/scripts: Use spelling 'utf-8' consistently

find python scripts -name '*.py' -exec sed -i -e "s/'utf8'/'utf-8'/g" {} \;

'utf-8' and 'utf8' are synonymous.

We used 'utf8' much more frequently than 'utf-8':

grep -e "'utf-8'" -r python scripts | wc -l
49
grep -e "'utf8'" -r python scripts | wc -l
71

However, the official python documentation uses 'utf-8' in their examples and we should be consistent.