[python3] use inspect.getfullargspec instead of inspect.getargspec The latter is now deprecated

[py3] ensure AjaxController return bytes publish() must return bytes, when the @ajaxfunc decorated method return None, this raised: TypeError: You cannot set Response.body to a text object (use Response.text)

[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.

Depend on cubicweb-file >= 2.0.0 I applied V1 of the patch, but V2 contained this extra hunk...

[test] Update webconfig's tests accounting for cubicweb-file being now a package WebconfigTC.test_locate_all_files and WebconfigTC.test_locate_resource assumed that cubicweb-file was installed in <share>/cubes/file whereas, from the latest release, it is now a standard Python package thus installed in site-packages as cubicweb_file. We simply update base directory of these two tests.

[widgets.js] fix obvious bug detected by sonar

[views/uicfg] obvious bug detected by sonar

[style] Make toolsutils.py flake8-clean Run `autopep8 --max-line-length 100 -i cubicweb/toolsutils.py` and fixed unused imports.