ustat: fix lambda syntax on python3.

$ ./ustat
  File "./ustat", line 242
    counts = sorted(counts.items(), key=lambda (_, v):
                                               ^
SyntaxError: invalid syntax

tools/ustat.py

@@ -239,10 +239,10 @@ class Tool(object):
             counts.update(probe.get_counts(self.bpf))
             targets.update(probe.targets)
         if self.args.sort:
-            counts = sorted(counts.items(), key=lambda (_, v):
+            counts = sorted(counts.items(), key=lambda _, v:
                             -v.get(self.args.sort.upper(), 0))
         else:
-            counts = sorted(counts.items(), key=lambda (k, _): k)
+            counts = sorted(counts.items(), key=lambda k, _: k)
         for pid, stats in counts:
             print("%-6d %-20s %-10d %-6d %-10d %-8d %-6d %-6d" % (
                   pid, targets[pid][:20],