Update some python examples to python 3 syntax.

Update some sample code snippets in comments to use python 3 syntax.
12 jobs for py3-2-20200613 in 36 minutes and 54 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #8789335
cvmfs
build:gcc8:opt

00:14:50

passed #8789337
cvmfs
build:gcc9:dbg

00:19:01

passed #8789336
cvmfs
build:gcc9:opt

00:12:09

passed #8789338
cvmfs
build:gcc9:opt:python3

00:14:29

passed #8789339
check-formatting

00:00:41

 
  Test
passed #8789340
check-copyright

00:00:59

passed #8789341
cvmfs
test:gcc8:opt

00:07:00

failed #8789343
cvmfs
test:gcc9:dbg

00:17:52

passed #8789342
cvmfs
test:gcc9:opt

00:07:36

passed #8789344
cvmfs
test:gcc9:opt:python3

00:05:53

passed #8789345
cvmfs
test_public_headers_build

00:03:13

passed #8789346
test-python3

00:01:09

 
Name Stage Failure
failed
test:gcc9:dbg Test
	143 - GaudiExamples.gaudiconfig2.histograms (Failed)
238 - GaudiHive.experiments.atlas.mc_reco_full_precedence_dump (Failed)
================================================================================
Running after_script
Uploading artifacts for failed job
Uploading artifacts...
test_report: found 3428 matching files

Uploading artifacts to coordinator... ok
id=8789343 responseStatus=201 Created token=TWNqEkRp
ERROR: Job failed: exit code 1