Force everything to use python3 for consistency.

Rather than "whatever people have installed as 'python' on their machine".

I've removed check-symbols.py because that's been broken for years and
we never even noticed, and I'm not sure it's worth fixing.

Test: treehugger, manual
Change-Id: Ieb996bbdf790a18d4b1fb46a409cc240ba2a2a49
diff --git a/libc/kernel/tools/clean_header.py b/libc/kernel/tools/clean_header.py
index 2c2d001..f86d36e 100755
--- a/libc/kernel/tools/clean_header.py
+++ b/libc/kernel/tools/clean_header.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 #------------------------------------------------------------------------------
 # Description of the header clean process
diff --git a/libc/kernel/tools/cpp.py b/libc/kernel/tools/cpp.py
index 8538eb0..9f53453 100755
--- a/libc/kernel/tools/cpp.py
+++ b/libc/kernel/tools/cpp.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """A glorified C pre-processor parser."""
 
 import ctypes
diff --git a/libc/kernel/tools/update_all.py b/libc/kernel/tools/update_all.py
index cef24dd..d878ec6 100755
--- a/libc/kernel/tools/update_all.py
+++ b/libc/kernel/tools/update_all.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 import sys, cpp, kernel, glob, os, re, getopt, clean_header, subprocess, shutil
 from defaults import *