Remove dependency on future

Sole use was raise_from. 

six.raise_from loses exception detail on Python 2, so god help the programmers of the past.

Fixes #196.

requirements/base.txt

@@ -2,7 +2,6 @@ coreapi>=2.3.3
 coreschema>=0.0.4
 ruamel.yaml>=0.15.34
 inflection>=0.3.1
-future>=0.16.0
 six>=1.10.0
 uritemplate>=3.0.0
 

src/drf_yasg/codecs.py

@@ -1,4 +1,4 @@
-from future.utils import raise_from
+from six import raise_from
 
 import copy
 import json

tox.ini

@@ -60,7 +60,7 @@ not_skip = __init__.py
 atomic = true
 multi_line_output = 5
 line_length = 120
-known_future_library = future,six
+known_future_library = six
 known_standard_library =
     collections,copy,distutils,functools,inspect,io,json,logging,operator,os,pkg_resources,re,setuptools,sys,
     types,warnings