interfaces: use triple quotes for Attribute value

Otherwise the source transformer converts it to bytes and
test-check-interfaces.py complains on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D6919

mercurial/interfaces/dirstate.py

@@ -22,7 +22,7 b' class idirstate(interfaceutil.Interface)'
 
     # TODO: all these private methods and attributes should be made
     # public or removed from the interface.
-    _ignore = interfaceutil.Attribute('Matcher for ignored files.')
+    _ignore = interfaceutil.Attribute("""Matcher for ignored files.""")
 
     def _ignorefiles():
         """Return a list of files containing patterns to ignore."""
@@ -30,8 +30,8 b' class idirstate(interfaceutil.Interface)'
     def _ignorefileandline(f):
         "Given a file `f`, return the ignore file and line that ignores it."
 
-    _checklink = interfaceutil.Attribute('Callable for checking symlinks.')
-    _checkexec = interfaceutil.Attribute('Callable for checking exec bits.')
+    _checklink = interfaceutil.Attribute("""Callable for checking symlinks.""")
+    _checkexec = interfaceutil.Attribute("""Callable for checking exec bits.""")
 
     @contextlib.contextmanager
     def parentchange():