Build on ubuntu 20.04

Author: ubergeek42

debian/control

@@ -5,7 +5,7 @@ Maintainer: DOMjudge Developers <[email protected]>
 Uploaders: Jaap Eldering <[email protected]>, Keith Johnson <[email protected]>, Thijs Kinkhorst <[email protected]>
 Build-Depends: libcurl4-gnutls-dev, libmagic-dev,
  debhelper (>= 12), libcgroup-dev, zip, libjsoncpp-dev
-Build-Depends-Indep: python-sphinx, python-sphinx-rtd-theme, rst2pdf, fontconfig, latexmk
+Build-Depends-Indep: python3-sphinx, python3-sphinx-rtd-theme, rst2pdf, fontconfig, latexmk
 Homepage: https://www.domjudge.org
 Standards-Version: 4.5.0
 Vcs-Git: https://github.com/DOMjudge/domjudge-packaging.git