Do the possible encoding to utf-8 in one spot.

Do this before calling a html_to_pdf transform method.
No longer convert everything to ascii for pisa.
First try to import pisa from the newer xhtml2pdf library.

Note that it can be hard for both libraries, including their
dependencies, to find versions that work on Python 2.4 (Plone 3).
Generally, wkhtmltopdf is recommended.