[UpdateTestChecks] Change shebang from python to python3

'python' means Python 2 on some platforms while Python 3 on others.
'python3' is Python 3 only. Python 2.7 End of Life is set to January 1,
2020. Getting rid of Python 2 support reduces maintenance burden.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D70730