Update tools to python3 (#85)

This updates the scripts in the tools directory to work with python3, python2
support is now deprecated.

The only significant API change is that the subprocess module works in bytes
instead of str, the rest are mainly style changes.
diff --git a/tools/generate_test_trace_a64_reference.py b/tools/generate_test_trace_a64_reference.py
index cef06fb..d42594d 100755
--- a/tools/generate_test_trace_a64_reference.py
+++ b/tools/generate_test_trace_a64_reference.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python3
 
 # Copyright 2016, VIXL authors
 # All rights reserved.
@@ -55,7 +55,7 @@
   # Find the trace tests.
   status, output = util.getstatusoutput(args.runner + ' --list')
   if status != 0: util.abort('Failed to list all tests')
-  tests = filter(lambda t: 'TRACE_' in t, output.split())
+  tests = [t for t in output.split() if 'TRACE_' in t]
   tests.sort()
 
   if not os.path.exists(args.outdir):
@@ -63,7 +63,7 @@
 
   for test in tests:
     # Run each test.
-    print 'Generating trace for ' + test;
+    print('Generating trace for ' + test);
     cmd = ' '.join([args.runner, '--generate_test_trace', test])
     status, output = util.getstatusoutput(cmd)
     if status != 0: util.abort('Failed to run ' + cmd + '.')