[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

utils/update_analyze_test_checks.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A script to generate FileCheck statements for 'opt' analysis tests.
 

utils/update_llc_test_checks.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A test case update script.
 

utils/update_mca_test_checks.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A test case update script.
 

utils/update_mir_test_checks.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Updates FileCheck checks in MIR tests.
 

utils/update_test_checks.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """A script to generate FileCheck statements for 'opt' regression tests.