Well... I have gone through Building_On_Linux several times making sure I have everything installed so if there is more required I'm not sure how to know what that is. (And the 18.3-system worked for building several Sigil-versions.)
Could there be some paths that need to be set? (Or something that the upgrade could have changed?)
I have searched for others asking about pythonproblems after the upgrade, but so far not finding anything.
I'm probably doing something stupid as usual...