Use python3 shebang for all python scripts

The `python` command may not exist on a system with Python 3 installed.
See https://peps.python.org/pep-0394/ for a discussion of the commands
expected to be installed.

Since Python2 was officially sunsetted almost three years ago
(https://www.python.org/doc/sunset-python-2/), IWYU should prefer
compatibility with newer systems over older ones.

Python code in scripts is still Python3/Python2-compatible, but we will no
longer make an effort to preserve Python2 support over time.

Fixes #1096.

fix_includes.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- fix_includes.py - rewrite source files based on iwyu output ------===##
 #

fix_includes_test.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- fix_includes_test.py - test for fix_includes.py ------------------===##
 #

iwyu-check-license-header.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- iwyu-check-license-header.py - check license headers -------------===##
 #

iwyu_test_util.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- iwyu_test_util.py - include-what-you-use test framework ----------===##
 #

iwyu_tool.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- iwyu_tool.py -----------------------------------------------------===##
 #

iwyu_tool_test.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- iwyu_tool_test.py - test for iwyu_tool.py ------------------------===##
 #

mapgen/iwyu-mapgen-cpython.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- iwyu-mapgen-cpython.py -------------------------------------------===##
 #

mapgen/iwyu-mapgen-qt.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- iwyu-mapgen-qt.py ------------------------------------------------===##
 #

run_iwyu_tests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- run_iwyu_tests.py - include-what-you-use test framework driver ---===##
 #
@@ -52,7 +52,7 @@ def TestIwyuOnRelevantFiles(filename):
                glob.glob('%s/*/%s-*' % (dirname, basename)) +
                glob.glob('%s.h' % all_but_extension) +
                glob.glob('%s/*/%s.h' % (dirname, basename)))
-  
+
   files_to_check = [f for f in all_files if not f.endswith(extension)]
   files_to_check.append(filename)
 

scrub-logs.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 ##===--- scrub-logs.py - generate README from Wiki sources ----------------===##
 #