Fix selfloop_edges AttributeError in algo/graph/mixin_helpers.py

When running the test
`wbia/algo/graph/mixin_dynamic.py::NonDynamicUpdate.apply_nondynamic_update:0`:

```
Traceback (most recent call last):
  File "/virtualenv/env3/lib/python3.6/site-packages/xdoctest/doctest_example.py", line 556, in run
    exec(code, test_globals)
  File "<doctest:/wbia/wildbook-ia/wbia/algo/graph/mixin_dynamic.py::NonDynamicUpdate.apply_nondynamic_update:0>", line rel: 9, abs: 1298, in <module>
    >>> infr.assert_neg_metagraph()
  File "/wbia/wildbook-ia/wbia/algo/graph/mixin_helpers.py", line 716, in assert_neg_metagraph
    [ne[0] for ne in list(infr.neg_metagraph.selfloop_edges())]
AttributeError: 'NiceGraph' object has no attribute 'selfloop_edges'
```

This is because networkx has moved some methods from the base graph class into
the main namespace.  See
https://networkx.github.io/documentation/stable/release/migration_guide_from_1.x_to_2.0.html.

So need to change

```
infr.neg_metagraph.selfloop_edges()
```

to

```
nx.selfloop_edges(infr.neg_metagraph)
```

wbia/algo/graph/mixin_helpers.py

@@ -713,7 +713,7 @@ def assert_neg_metagraph(infr):
 
         # Self loops should correspond to the number of inconsistent components
         neg_self_loop_nids = sorted(
-            [ne[0] for ne in list(infr.neg_metagraph.selfloop_edges())]
+            [ne[0] for ne in list(nx.selfloop_edges(infr.neg_metagraph))]
         )
         incon_nids = sorted(infr.nid_to_errors.keys())
         assert neg_self_loop_nids == incon_nids