Switch builddep to python3 for debian unstable.

debian/control

@@ -27,7 +27,7 @@ Build-Depends: debhelper (>= 9),
                libsdl2-dev,
                cmake,
                wget,
-               python,
+               python3,
                g++,
                qtwebengine5-dev,
                qtquickcontrols2-5-dev,