Call python3.bat explicitly from gn.bat on Windows.
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 <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@google.com>
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@google.com>

CoherentLabs/depot_tools,CoherentLabs/depot_tools