Fix StochKit export on Python 3 with lxml installed (#375)
Fix StochKit export on Python 3 with lxml installed (#375)

This previously failed because lxml outputted bytes, which
needs conversion into a UTF-8 string on Python 3.
alubbock/pysb,LoLab-VU/pysb,johnbachman/pysb,pysb/pysb,jmuhlich/pysb,LoLab-VU/pysb,johnbachman/pysb