undumprevlog: update to valid Python 3 syntax

I didn't do anything to ensure correctness here, just enough to avoid
tracebacks in the import checker, which uses the native ast module to
try and parse all our Python files.

contrib/undumprevlog

@@ -3,7 +3,7 b''
 # $ hg init
 # $ undumprevlog < repo.dump
 
-from __future__ import absolute_import
+from __future__ import absolute_import, print_function
 
 import sys
 from mercurial import (
@@ -27,7 +27,7 b' while True:'
     if l.startswith("file:"):
         f = l[6:-1]
         r = revlog.revlog(opener, f)
-        print f
+        print(f)
     elif l.startswith("node:"):
         n = node.bin(l[6:-1])
     elif l.startswith("linkrev:"):