python: Replace os.system with subprocess.call.

Generally, we avoid os.system, since it shells out and thus can be
a cause of security issues.

tools/lib/pretty_print.py

@@ -9,7 +9,7 @@ from .template_parser import (
     is_django_block_tag,
 )
 from six.moves import range
-import os
+import subprocess
 
 def pretty_print_html(html, num_spaces=4):
     # type: (str, int) -> str
@@ -181,6 +181,6 @@ def validate_indent_html(fn):
         temp_file.write(phtml)
         temp_file.close()
         print('Invalid Indentation detected in file: %s\nDiff for the file against expected indented file:' % (fn))
-        os.system('diff %s %s' % (fn, '/var/tmp/pretty_html.txt'))
+        subprocess.call(['diff', fn, '/var/tmp/pretty_html.txt'], stderr=subprocess.STDOUT)
         return 0
     return 1