Update GN to require a minimum of python 3.9

Python 3.9 is officially EOL already, so I'd like to raise it higher, but it's still bundled with xcode on mac.

Bug: None
Change-Id: I6cc6d63d45401e023b916bfe5460a6c56a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/23300
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: Matt Stark <msta@google.com>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 736f609..ba3184c 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -13,13 +13,7 @@
 import subprocess
 import sys
 
-# IMPORTANT: This script is also executed as python2 on
-# GN's CI builders.
-
-try:  # py3
-  from shlex import quote as shell_quote
-except ImportError:  # py2
-  from pipes import quote as shell_quote
+assert sys.version_info >= (3, 9, 0)
 
 SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
 REPO_ROOT = os.path.dirname(SCRIPT_DIR)
@@ -152,7 +146,7 @@
     if platform.system() == "Windows":
       return ' '.join(result)
     else:
-      return ' '.join(shell_quote(item) for item in result)
+      return ' '.join(shlex.quote(item) for item in result)
 
 
 def main(argv):