refactor: no need for six anymore

Author: nedbat

.isort.cfg

@@ -4,4 +4,4 @@ force_grid_wrap=0
 include_trailing_comma=True
 line_length=79
 multi_line_output=3
-known_third_party = coverage,django,setuptools,six,unittest_mixins
+known_third_party = coverage,django,setuptools,unittest_mixins

setup.py

@@ -64,7 +64,6 @@ def read(*names, **kwargs):
     packages=['django_coverage_plugin'],
     install_requires=[
         'coverage',
-        'six >= 1.4.0',
     ],
     license='Apache-2.0',
     classifiers=classifiers.splitlines(),