Use python3 in all run lines

We already did this in some, this converts the rest.
Also chmod +x on write_fake_manifests.py while here.

configure.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright 2001 Google Inc. All Rights Reserved.
 #
@@ -19,8 +19,6 @@
 Projects that use ninja themselves should either write a similar script
 or use a meta-build system that supports Ninja output."""
 
-from __future__ import print_function
-
 from optparse import OptionParser
 import os
 import pipes

misc/measure.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright 2011 Google Inc. All Rights Reserved.
 #
@@ -17,8 +17,6 @@
 """measure the runtime of a command by repeatedly running it.
 """
 
-from __future__ import print_function
-
 import time
 import subprocess
 import sys

misc/ninja_syntax_test.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright 2011 Google Inc. All Rights Reserved.
 #

misc/write_fake_manifests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Writes large manifest files, for manifest parser performance testing.
 

src/browse.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright 2001 Google Inc. All Rights Reserved.
 #
@@ -20,8 +20,6 @@
 it when needed.
 """
 
-from __future__ import print_function
-
 try:
     import http.server as httpserver
     import socketserver