gitlab-ci.yml: use python3 in before_script

I don't really know what this is used to since it's not related to a
version test. Also, not sure "pip3" will exist.
12 jobs for master in 1 minute and 12 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #23691
build:2.7

00:00:16

failed #23692
build:3

00:01:11

 
  Test
skipped #23694
allowed to fail
lint:2.7
skipped #23698
allowed to fail
lint:3
skipped #23693
pyflakes:2.7
skipped #23697
pyflakes:3
skipped #23695
unittests:2.7
skipped #23699
unittests:3
skipped #23696
allowed to fail
unittests-modules:2.7
skipped #23700
allowed to fail
unittests-modules:3
 
  Deploy
skipped #23701
doc:2.7
skipped #23702
doc:3
 
Name Stage Failure
failed
build:2.7 Build
Checking out 3b8cc43d as master...
Removing build/
Removing dist/
Removing weboob.egg-info/

Skipping Git submodules setup
$ pip3 install -r .ci/requirements.txt
/bin/bash: line 82: pip3: command not found
ERROR: Job failed: exit code 1
failed
build:3 Build

Installing weboob applications into ‘/root/bin’.
Traceback (most recent call last):
File "./tools/local_install.py", line 30, in <module>
subprocess.check_call([sys.executable, os.path.join(dest, 'weboob-config'), 'update'])
File "/usr/local/lib/python3.8/subprocess.py", line 364, in check_call
raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/usr/local/bin/python3', '/root/bin/weboob-config', 'update']' returned non-zero exit status 130.
ERROR: Job failed: exit code 1