Again ... I'm not familiar with the pclos package-naming standard, but can you verify that you've actually installed the python 3 module of lxml? python-lxml very-well could be the python 3 package on pclos (it is on Arch), but on others it might be the python 2 version of the module. I'm just being thorough, not condescending!
Just to clarify, though: you've not had
any success with running Sigil on PCLOS yet?