Fix anytree version upgrade issues

[vorpal-buildbot]

Reported on Discord by bakert#2193

[bakert]

/usr/local/lib/python3.7/site-packages/py/_path/local.py:668: in pyimport
    __import__(modname)
decksite/__init__.py:38: in <module>
    from . import api as API, admin # isort:skip # pylint: disable=wrong-import-position, unused-import
decksite/api.py:5: in <module>
    from decksite import APP, auth, league
decksite/league.py:10: in <module>
    from decksite.data import competition, deck, match, person, query
decksite/data/competition.py:6: in <module>
    from decksite.data import archetype, deck, query
decksite/data/archetype.py:15: in <module>
    class Archetype(Container, NodeMixin):
E   TypeError: multiple bases have instance lay-out conflict

[bakert]

Asked the anytree devs at c0fec0de/anytree#77

[bakert]

I raised this with the Munch devs, too – Infinidat/munch#39 – although I'm less hopeful that they'll see it as anything to do with them than anytree.

[bakert]

Much devs believe that because Munch derives from dict it uses __slots__ and NodeMixin probably now does same. I wonder if the right answer here is to freeze anytree version forever.

[bakert]

Got a reply from anytree too but no path forward better than freezing anytree version so far.

Can also make the node behavior hang off a Container.node attribute instead of being mixed in but that's an irritating rewrite.

[bakert]

Nothing from anytree. Not really any good paths forward here. Everything works at least.

[bakert]

No anytree release since 2.6.0.

[bakert]

anytree devs fixed it in 2.7.0!