Update cpplint.py's interpreter directive to python3.
Update cpplint.py's interpreter directive to python3.

Change-Id: Ic6c1ebd049350348cdd0cd4db717bda8c6f3dc34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3540610
Reviewed-by: Aravind Vasudevan <dbe94b625b74f03bb5e24a737a4f1e15753433de@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

CoherentLabs/depot_tools,CoherentLabs/depot_tools