make nox session=docs fails on darwin

[skatromb]

doctest fails on darwin platform

**********************************************************************
File "../../lib/python3.13/site-packages/pyroute2/ndb/main.py", line ?, in netns
Failed example:
    nslist = ['netns01',
              'netns02',
              'netns03']

    for nsname in nslist:
        ndb.sources.add(netns=nsname)

    report = ndb.addresses.summary()
    report.select_records(target=lambda x: x.startswith('netns'))
    report.select_fields('address', 'ifname', 'target')
    for line in report.format('json'):
        print(line)
Exception raised:
    Traceback (most recent call last):
      File "~/.asdf/installs/python/3.13.1/lib/python3.13/doctest.py", line 1395, in __run
        exec(compile(example.source, filename, "single",
        ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                     compileflags, True), test.globs)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "<doctest netns[0]>", line 6, in <module>
        ndb.sources.add(netns=nsname)
        ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
      File "/Users/skatromb/code/pyroute2/.nox-skatromb/docs/lib/python3.13/site-packages/pyroute2/ndb/view.py", line 504, in add
        spec = dict(Source.defaults(spec))
                    ~~~~~~~~~~~~~~~^^^^^^
      File "/Users/skatromb/code/pyroute2/.nox-skatromb/docs/lib/python3.13/site-packages/pyroute2/ndb/source.py", line 238, in defaults
        ret['netns'] = netns._get_netnspath(spec['netns'])
                       ^^^^^
    NameError: name 'netns' is not defined. Did you mean: 'NetNS'?