Repair Python executable path in the bootstrap script

The previous version failed in some cases when the bootstrap script was
executed with "python3" and "python" was not available in the system.

bootstrap.py

@@ -62,6 +62,14 @@ def set_default_subparser(self, name, args=None):
 argparse.ArgumentParser.set_default_subparser = set_default_subparser
 
 
+def get_python_path():
+    """Return the path to Python."""
+    path = sys.executable  # maybe python2 or python3
+    if not path:
+        path = 'python'
+    return path
+
+
 def get_script_path():
     """Return the path to the this script."""
     return __file__
@@ -179,9 +187,11 @@ def command_ci_lint(args):
     if not subcommands:
         subcommands.append('cpp')
 
+    python_path = get_python_path()
     script_path = get_script_path()
     verbose_opt = ['-v'] if args.verbose else []
-    run(['python', script_path, 'clean'] + verbose_opt, verbose=args.verbose)
+    run([python_path, script_path, 'clean'] + verbose_opt,
+        verbose=args.verbose)
 
     for cmd in subcommands:
         globals()['_command_ci_lint_' + cmd](args)
@@ -218,19 +228,21 @@ def _command_ci_lint_python(args):
 
 def command_ci_test(args):
     """Run tests for CI."""
+    python_path = get_python_path()
     script_path = get_script_path()
     verbose_opt = ['-v'] if args.verbose else []
 
-    run(['python', script_path, 'clean'] + verbose_opt, verbose=args.verbose)
+    run([python_path, script_path, 'clean'] + verbose_opt,
+        verbose=args.verbose)
 
-    run(['python', script_path, 'init', 'strict', 'debug'] + args.args +
+    run([python_path, script_path, 'init', 'strict', 'debug'] + args.args +
         verbose_opt, verbose=args.verbose)
     run(['cmake', '--build', '.', '--target', 'all'], verbose=args.verbose)
     if cmake_has_target('check'):
         run(['cmake', '--build', '.', '--target', 'check'],
             verbose=args.verbose)
 
-    run(['python', script_path, 'init', 'strict', 'release'] + args.args +
+    run([python_path, script_path, 'init', 'strict', 'release'] + args.args +
         verbose_opt, verbose=args.verbose)
     run(['cmake', '--build', '.', '--target', 'all'], verbose=args.verbose)
     if cmake_has_target('check'):
@@ -242,8 +254,8 @@ def command_ci_test(args):
             verbose=args.verbose)
 
     if cmake_has_target('install'):
-        run(['python', script_path, 'init', 'strict', 'release',
-            'test-install'] + args.args + verbose_opt, verbose=args.verbose)
+        run([python_path, script_path, 'init', 'strict', 'release',
+             'test-install'] + args.args + verbose_opt, verbose=args.verbose)
         run(['cmake', '--build', '.', '--target', 'install'],
             verbose=args.verbose)