[gn build] Change python run lines to python3

macOS 12.3 no longer ships non-3 python.

Almost all of these scripts were launched by ninja, and the GN files
already told it to run them under python3, so this is a fairly small
change.  The main effect is that if you run them manually, you now
get the same behavior.

(A small set of scripts, gn.py, gen.py, sync_source_lists_from_cmake.py,
are for manual running.  For these, it is an actual change.)

Differential Revision: https://reviews.llvm.org/D122345

llvm/utils/gn/build/run_tablegen.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Runs tablegen."""
 
 import subprocess

llvm/utils/gn/build/symbol_exports.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Converts a .exports file to a format consumable by linkers.
 

llvm/utils/gn/build/symlink_or_copy.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Symlinks, or on Windows copies, an existing file to a second location.
 

llvm/utils/gn/build/sync_source_lists_from_cmake.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Helps to keep BUILD.gn files in sync with the corresponding CMakeLists.txt.
 
@@ -14,8 +14,6 @@ commit for each cmake commit this merges. If an error is reported, the state
 of HEAD is unspecified; run `git reset --hard origin/main` if this happens.
 """
 
-from __future__ import print_function
-
 from collections import defaultdict
 import os
 import re

llvm/utils/gn/build/write_cmake_config.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 r"""Emulates the bits of CMake's configure_file() function needed in LLVM.
 
 The CMake build uses configure_file() for several things.  This emulates that
@@ -32,8 +32,6 @@ Fails if any of the KEY=VALUE arguments aren't needed for processing the
 input file, or if the input file references keys that weren't passed in.
 """
 
-from __future__ import print_function
-
 import argparse
 import os
 import re

llvm/utils/gn/build/write_library_dependencies.py

@@ -1,5 +1,4 @@
-#!/usr/bin/env python
-from __future__ import print_function
+#!/usr/bin/env python3
 
 import argparse
 import os

llvm/utils/gn/build/write_vcsrevision.py

@@ -1,9 +1,7 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Gets the current revision and writes it to VCSRevision.h."""
 
-from __future__ import print_function
-
 import argparse
 import os
 import subprocess

llvm/utils/gn/get.py

@@ -1,17 +1,12 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Downloads a prebuilt gn binary to a place where gn.py can find it."""
 
 from __future__ import print_function
 
 import io
 import os
-try:
-    # In Python 3, we need the module urllib.reqest. In Python 2, this
-    # functionality was in the urllib2 module.
-    from urllib import request as urllib_request
-except ImportError:
-    import urllib2 as urllib_request
 import sys
+import urllib.request
 import zipfile
 
 
@@ -19,7 +14,7 @@ def download_and_unpack(url, output_dir, gn):
     """Download an archive from url and extract gn from it into output_dir."""
     print('downloading %s ...' % url, end='')
     sys.stdout.flush()
-    data = urllib_request.urlopen(url).read()
+    data = urllib.request.urlopen(url).read()
     print(' done')
     zipfile.ZipFile(io.BytesIO(data)).extract(gn, path=output_dir)
 

llvm/utils/gn/gn.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Calls `gn` with the right --dotfile= and --root= arguments for LLVM."""
 
 # GN normally expects a file called '.gn' at the root of the repository.

llvm/utils/gn/secondary/libcxx/utils/gen_link_script.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

llvm/utils/gn/secondary/llvm/include/llvm/Support/write_extension_def.py

@@ -1,5 +1,4 @@
-#!/usr/bin/env python
-from __future__ import print_function
+#!/usr/bin/env python3
 
 import argparse
 import os

llvm/utils/gn/secondary/llvm/tools/llvm-config/write_extension_dependencies.py

@@ -1,8 +1,6 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 r"""Writes ExtensionDepencencies.inc."""
 
-from __future__ import print_function
-
 import argparse
 import os
 import re