The pip command wants the Python package name, not the Ubuntu package name.
Try the command python3.6 -m pip install PyQt5
You should be able to see available related packages with python3.6 -m pip search pyqt
this is the epoch number.
It somehow overrides the version numbers: 3.0.0 < 1:1.0.0 < 2:0.1.0 etc. This would allow developers to change their numbering scheme but keep the updates going. yum mostly prints the epoch, but doesn't accept it as input on most of his commands. It can be a little bit disturbing, yes :)