Fixed compatability issues: set literals don't exist in py26 and __builtin__ doesn't exist in py3.

Author: Nurdok

test_pep257.py

@@ -21,13 +21,13 @@ def test_ignore_list():
     no blank line after one-liner is bad. Also this - \"\"\"
     return foo
     """
-    expected_error_codes = {'D100', 'D400', 'D401', 'D205', 'D209'}
+    expected_error_codes = set(('D100', 'D400', 'D401', 'D205', 'D209'))
     mock_open = mock.mock_open(read_data=function_to_check)
-    with mock.patch('__builtin__.open', mock_open, create=True):
+    with mock.patch('pep257.open', mock_open, create=True):
         errors = tuple(pep257.check(['filepath']))
         error_codes = set(error.code for error in errors)
         assert error_codes == expected_error_codes
 
         errors = tuple(pep257.check(['filepath'], ignore=['D100', 'D202']))
         error_codes = set(error.code for error in errors)
-        assert error_codes == expected_error_codes - {'D100', 'D202'}
+        assert error_codes == expected_error_codes - set(('D100', 'D202'))

tox.ini

@@ -7,7 +7,7 @@
 envlist = py26, py27, py32, py33, py34, pypy, pypy3
 
 [testenv]
-commands = py.test --pep8
+commands = py.test --pep8 --clearcache
 deps = pytest
        pytest-pep8
        mock