Use six to solve raise from issue in Python 2.7.

Author: Nurdok

setup.py

@@ -28,6 +28,7 @@
     package_data={'pydocstyle': ['data/*.txt']},
     install_requires=[
         'snowballstemmer',
+        'six',
     ],
     entry_points={
         'console_scripts': [

src/pydocstyle/parser.py

@@ -1,10 +1,9 @@
 """Python code parser."""
 
 import logging
-import sys
+import six
 import textwrap
 import tokenize as tk
-from collections import defaultdict
 from itertools import chain, dropwhile
 from re import compile as re
 
@@ -277,7 +276,7 @@ def parse(self, filelike, filename):
         try:
             compile(src, filename, 'exec')
         except SyntaxError as error:
-            raise ParseError() from error
+            six.raise_from(ParseError(), error)
         self.stream = TokenStream(StringIO(src))
         self.filename = filename
         self.all = None