Fix the pyxattr dependency to allow kiwi to function

[Conan-Kudo]

Most Linux distributions offer the pyxattr module, including openSUSE Tumbleweed. Going forward, we will use the pyxattr module by default as a dependency and only switch back to the
other xattr module when on older SUSE Linux distributions that lack the pyxattr module.

Note that because kiwi uses setuptools to create the CLI entry points, kiwi checks the Python dependencies before executing, so we change the dependency in the setup.py accordingly so that it will not fail to start.

[Conan-Kudo]

@schaefi Note, this is intended to fix the breakage we're seeing in testing on CentOS without breaking SUSE:

[   75s] cd /usr/src/packages/SOURCES && rm -rf /usr/src/packages/KIWI-oem && LANG=en_US.UTF-8 /usr/bin/kiwi --debug  --profile Disk system build --description /usr/src/packages/SOURCES --target-dir /usr/src/packages/KIWI-oem 
[   75s] Traceback (most recent call last):
[   75s]   File "/usr/bin/kiwi", line 6, in <module>
[   75s]     from pkg_resources import load_entry_point
[   75s]   File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3095, in <module>
[   75s]     @_call_aside
[   75s]   File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3079, in _call_aside
[   75s]     f(*args, **kwargs)
[   75s]   File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3108, in _initialize_master_working_set
[   75s]     working_set = WorkingSet._build_master()
[   75s]   File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 570, in _build_master
[   75s]     ws.require(__requires__)
[   75s]   File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 888, in require
[   75s]     needed = self.resolve(parse_requirements(requirements))
[   75s]   File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 774, in resolve
[   75s]     raise DistributionNotFound(req, requirers)
[   75s] pkg_resources.DistributionNotFound: The 'xattr' distribution was not found and is required by kiwi

[schaefi]

Thanks much, I was just looking at the failed tests but you already fixed it :) very cool