[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.
 
@@ -45,7 +45,7 @@ def main():
   for test in test_paths:
     with open(test) as f:
       input_lines = [l.rstrip() for l in f]
-    
+
     first_line = input_lines[0] if input_lines else ""
     if 'autogenerated' in first_line and script_name not in first_line:
       common.warn("Skipping test which wasn't autogenerated by " + script_name, test)

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.