[UpdateTestChecks] Change shebang from python to python3

[MaskRay]

Summary

'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.

[lebedev.ri]

Can do this change for everything llvm/utils/update_*-related at once?

[MaskRay]

Update all update_*_checks.py

[lebedev.ri]

I'm okay with this.

llvm/utils/update_mca_test_checks.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A test case update script.
 
 This script is a utility to update LLVM 'llvm-mca' based test cases with new
 FileCheck patterns.
 """
 
 import argparse