lxml: replace deprecated etree getiterator/getchildren

These methods were deprecated in stdlib xml.etree and also in lxml, and
are ultimately removed in python 3.9.
7 jobs for master in 25 minutes and 45 seconds (queued for 3 minutes and 34 seconds)
Status Job ID Name Coverage
  Build
passed #28903
build:3

00:03:56

 
  Test
passed #28906
load-modules:3

00:04:32

passed #28904
pyflakes:3

00:05:00

passed #28905
pyflakes-strict:3

00:04:47

passed #28907
unittests:3

00:04:05

43.0%
failed #28908
allowed to fail
unittests-modules:3

00:13:43

42.0%
 
  Deploy
passed #28909
doc:3

00:03:10

 
Name Stage Failure
failed
unittests-modules:3 Test
        </head>
<body>
<h1>Error: 400 Bad Request</h1>
<p>Sorry, the requested URL <tt>'https://ci.weboob.org/api/v1/modules'</tt>
caused an error:</p>
<pre>Invalid JSON payload.</pre>
</body>
</html>
ERROR: Job failed: exit code 1