lock: pass sysstr to warnings module

This makes test-lock.py slightly less broken, but it's still pretty
far from passing.

Differential Revision: https://phab.mercurial-scm.org/D3503

mercurial/lock.py

@@ -204,7 +204,7 b' class lock(object):'
 
     def __del__(self):
         if self.held:
-            warnings.warn("use lock.release instead of del lock",
+            warnings.warn(r"use lock.release instead of del lock",
                     category=DeprecationWarning,
                     stacklevel=2)