change shebang to python3 (#383)

use python3 in the shebang

Upstream PR: https://github.com/cpplint/cpplint/pull/383

Bug: b:414869564
Test: `./cpplint.py --help` still works
Change-Id: If1c88af73364554135ae8272625ed73af43c4813
(cherry picked from commit b6ada002ca39c8b55ed08b278e0135c8a940db89)