Fix escaping in a test runner regex.

runtests.py

@@ -487,7 +487,7 @@ class build_ext(_build_ext):
 
 
 class ErrorWriter(object):
-    match_error = re.compile('(warning:)?(?:.*:)?\s*([-0-9]+)\s*:\s*([-0-9]+)\s*:\s*(.*)').match
+    match_error = re.compile(r'(warning:)?(?:.*:)?\s*([-0-9]+)\s*:\s*([-0-9]+)\s*:\s*(.*)').match
 
     def __init__(self):
         self.output = []