package/python-service-identity: the attrs distribution is required

package/python-service-identity: the attrs distribution is required

Apparently the service identify code requires the python attrs
to be availabe:

Traceback (most recent call last):
  File "/opt/exablox/bin/configsrv", line 5, in <module>
    from pkg_resources import load_entry_point
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 3138, in <module>
    @_call_aside
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 3124, in _call_aside
    f(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 3151, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 661, in _build_master
    ws.require(__requires__)
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 962, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 849, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'attrs' distribution was not found and is required by service-identity

Signed-off-by: Charles Hardin <ckhardin@exablox.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>