py-libxml2: Byte compile

This eliminates annoying fs-violation errors, notably from
textproc/itstool and textproc/gtk-doc where the byte code is generated
at build time instead of already having been generated by py-libxml2.

It is still peculiar to me that these files are dumped directly into
PYTHON_SITELIBDIR and don't have a subdirectory of their own.
Worth investigating, but hierachical changes are obviously more
intrusive and beyond this scope.