Fix minor python 2 -> 3 issue in doxygen script

13 jobs for python3-doxygen in 37 minutes and 45 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Level0
passed #13700
level0:lal

00:03:42

 
  Level1
passed #13701
level1:lalframe

00:02:27

passed #13702
level1:lalmetaio

00:01:10

passed #13703
level1:lalsimulation

00:06:12

passed #13704
level1:lalxml

00:01:49

 
  Level2
passed #13705
level2:lalburst

00:01:50

passed #13706
level2:lalinspiral

00:05:42

passed #13707
level2:lalpulsar

00:03:21

passed #13708
level2:lalstochastic

00:01:50

 
  Level3
passed #13709
level3:laldetchar

00:02:36

passed #13710
level3:lalinference

00:10:21

 
  Level4
passed #13711
level4:lalapps

00:09:53

 
  Container
passed #13712
dockerize

00:01:50