Use version of taxii2-client from PyPI.

.isort.cfg

@@ -1,7 +1,7 @@
 [settings]
 check=1
 diff=1
-known_third_party=dateutil,pytest,pytz,six,requests,taxii2_client
+known_third_party=dateutil,pytest,pytz,six,requests,taxii2client
 known_first_party=stix2
 not_skip=__init__.py
 force_sort_within_sections=1

setup.py

@@ -52,6 +52,7 @@ setup(
         'pytz',
         'requests',
         'simplejson',
-        'six'
+        'six',
+        'taxii2-client',
     ],
 )

stix2/test/test_data_sources.py

@@ -1,5 +1,5 @@
 import pytest
-from taxii2_client import Collection
+from taxii2client import Collection
 
 from stix2.sources import DataSource, Filter, taxii