restore python version warning

xsstrike.py

@@ -4,7 +4,6 @@ from __future__ import print_function
 
 # Let's import whatever we need from standard lib
 import argparse
-import concurrent.futures
 import copy
 import re
 
@@ -33,6 +32,7 @@ print('''%s
 %s''' % (red, white, end))
 
 try:
+    import concurrent.futures
     from urllib.parse import quote_plus, unquote, urlparse
 except ImportError:  # throws error in python2
     print('%s XSStrike isn\'t compatible with python2.\n Use python > 3.4 to run XSStrike.' % bad)