[py3] Let InstanceCommand.run_arg() always return an int
The caller of this method (which is method "run_args") does `max(status,
self.run_arg(appid))` and run_arg might return None if the underlying
<cmdname>_instance method returns None. On python3 max(1, None) breaks
with "TypeError: unorderable types: int() > NoneType()". So we enforce
run_arg() method to return an integer even if the underlying method
returns None.
This fixes call of "cubicweb-ctl i18ninstance" with Python3.
### Requirements for tests in various cubicweb/**/test directories. ###
## shared by several test folders
cubicweb-card == 0.5.8
docutils
Twisted < 16.0.0
webtest
## cubicweb/test
Pygments
mock
#fyzz XXX pip install fails
cubicweb-file == 1.18.0
cubicweb-localperms == 0.3.2
cubicweb-tag == 1.8.3
## cubicweb/devtools/test
flake8
## cubicweb/hooks/test
psycopg2
## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth
## cubicweb/sobject/test
cubicweb-comment == 1.12.2