reuse stack/slapos configuration
Remove version to use xz-friendly hexagonit.recipe.download provided by slapos stack
use latest version of slapos.recipe.build to cleanup pyc files
Pin spyne to 2.9.3

This should have been part of 55f915c3604df1e5a243b84a2a7cecf9c0ff99cf
pdftk removed from erp5
remove some remaining references to pdftk
use pyPdf instead of PyPDF2 until PyPDF2 can be installed
update md5sum of instance-zope.cfg.in
Merge remote-tracking branch 'origin/erp5' into erp5-z2.13