Use relative paths to avoid case issues on Windows

Author: lordmauve

src/tests/test_definitions.py

@@ -2,6 +2,7 @@
 
 import sys
 import os
+import os.path
 import re
 import pytest
 from pydocstyle.violations import Error, ErrorRegistry
@@ -59,7 +60,7 @@ def run_case(test_case):
                              fromlist=['expectation'],
                              level=1)
 
-    test_case_file = case_module.__file__
+    test_case_file = os.path.relpath(case_module.__file__)
     results = list(check([test_case_file],
                          select=set(ErrorRegistry.get_error_codes()),
                          ignore_decorators=re.compile(