util: call warnings.warn() with a sysstr in nouideprecwarn

This preserves nouideprecwarn wanting a bytes, which is consistent
with the rest of hg.

Differential Revision: https://phab.mercurial-scm.org/D2210

mercurial/util.py

@@ -253,7 +253,7 b' def nouideprecwarn(msg, version, stackle'
     if _dowarn:
         msg += ("\n(compatibility will be dropped after Mercurial-%s,"
                 " update your code.)") % version
-        warnings.warn(msg, DeprecationWarning, stacklevel + 1)
+        warnings.warn(pycompat.sysstr(msg), DeprecationWarning, stacklevel + 1)
 
 DIGESTS = {
     'md5': hashlib.md5,