Fix Travis CI build error caused by previous commit

.travis.yml

@@ -15,6 +15,7 @@ python:
 
 install:
   - python -m pip install --upgrade pip
+  - pip install -U importlib-metadata
   - pip install --upgrade setuptools
   - pip install -U tox-travis
   - pip install -U coveralls

nixnet/_utils.py

@@ -3,7 +3,7 @@
 from __future__ import print_function
 
 try:
-    from collections.abc import Iterable # python 3.3+
+    from collections.abc import Iterable  # python 3.3+
 except ImportError:
     from collections import Iterable  # python 2.7
 import typing  # NOQA: F401

nixnet/system/_collection.py

@@ -3,9 +3,11 @@
 from __future__ import print_function
 
 try:
-    from collections.abc import Iterable, Sized  # python 3.3+
+    from collections.abc import Iterable  # python 3.3+
+    from collections.abc import Sized  # python 3.3+
 except ImportError:
-    from collections import Iterable, Sized  # python 2.7
+    from collections import Iterable  # python 2.7
+    from collections import Sized  # python 2.7
 import typing  # NOQA: F401
 
 from nixnet import _cprops