Log Message:
add a build dependency on py-expat so the build doesn't fail with this message:

 failed to find and generate stubs for libxml2, aborting ...
 <class 'xml.sax._exceptions.SAXReaderNotAvailable'> No parsers found
 *** Error code 1
 Stop.
 make: stopped in /usr/pkgsrc/textproc/py-libxml2