Remove usage of getStringIO (#590)
Remove usage of getStringIO (#590)

reportlab 3.6.7 removed getStringIO and getByteIO: https://github.com/MrBitBucket/reportlab-mirror/commit/684d21e2da8f835ca3670e78cc9a011ba71a7e9a

This should resolve: Cannot import name 'getStringIO' from 'reportlab.lib.utils'  #589
xhtml2pdf/xhtml2pdf,xhtml2pdf/xhtml2pdf