Call python3.bat explicitly from gn.bat on Windows.

This ensures users running from a linux-style shell call through to
the right Python interpreter.

Bug: None
Change-Id: I2e5f2d1a02537ee86f682607670199d37225cf15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3168710
Auto-Submit: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

gn.bat

@@ -9,4 +9,4 @@ setlocal
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-python3 "%~dp0\gn.py" %*
+python3.bat "%~dp0\gn.py" %*