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

@@ -1,46 +1,46 b''
 #!/usr/bin/env python
 # Undump a dump from dumprevlog
 # $ hg init
 # $ undumprevlog < repo.dump
 
-from __future__ import absolute_import
+from __future__ import absolute_import, print_function
 
 import sys
 from mercurial import (
     node,
     revlog,
     transaction,
     util,
     vfs as vfsmod,
 )
 
 for fp in (sys.stdin, sys.stdout, sys.stderr):
     util.setbinary(fp)
 
 opener = vfsmod.vfs('.', False)
 tr = transaction.transaction(sys.stderr.write, opener, {'store': opener},
                              "undump.journal")
 while True:
     l = sys.stdin.readline()
     if not l:
         break
     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:"):
         lr = int(l[9:-1])
     elif l.startswith("parents:"):
         p = l[9:-1].split()
         p1 = node.bin(p[0])
         p2 = node.bin(p[1])
     elif l.startswith("length:"):
         length = int(l[8:-1])
         sys.stdin.readline() # start marker
         d = sys.stdin.read(length)
         sys.stdin.readline() # end marker
         r.addrevision(d, tr, lr, p1, p2)
 
 tr.close()