Update benchmarks for Python3 while retaining backwards compatibility with Python2

benchmark/catrank_proto.py

@@ -2,8 +2,12 @@
 
 from common import rand_int, rand_double, rand_bool, WORDS, from_bytes_helper
 from random import choice
-from string import letters
-from itertools import izip
+from string import ascii_letters
+try:
+  # Python 2
+  from itertools import izip
+except ImportError:
+  izip = zip
 import catrank_pb2
 
 class Benchmark:
@@ -22,7 +26,7 @@ class Benchmark:
             result = request.result.add()
             result.score = 1000 - i
             url_size = rand_int(100)
-            result.url = "http://example.com/" + ''.join([choice(letters) for _ in range(url_size)])
+            result.url = "http://example.com/" + ''.join([choice(ascii_letters) for _ in range(url_size)])
 
             isCat = rand_bool()
             isDog = rand_bool()