Use python3 for cpplint and get_toolchain on Win

R=gavinmak@google.com

Change-Id: I7d9ca9530640ffa4b8e60c247f809b0ff477d88e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3673716
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

cpplint.bat

@@ -8,4 +8,4 @@ setlocal
 :: standalone, but allow other PATH manipulations to take priority.
 set PATH=%PATH%;%~dp0
 
-python "%~dp0cpplint.py" %*
+python3 "%~dp0cpplint.py" %*

win_toolchain/get_toolchain_if_necessary.py

@@ -481,7 +481,7 @@ def main():
     # This script requires Windows Python, so invoke with depot_tools' Python.
     def winpath(path):
       return subprocess.check_output(['cygpath', '-w', path]).strip()
-    python = os.path.join(DEPOT_TOOLS_PATH, 'python.bat')
+    python = os.path.join(DEPOT_TOOLS_PATH, 'python3.bat')
     cmd = [python, winpath(__file__)]
     if args.output_json:
       cmd.extend(['--output-json', winpath(args.output_json)])