Fix isascii() method calls for python 3.6 (#4958)

* fix isascii for python3.6

* update comment with python 3.7 note

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/general.py

@@ -160,6 +160,11 @@ def is_pip():
     # Is file in a pip package?
     return 'site-packages' in Path(__file__).resolve().parts
 
+def is_ascii(s=''):
+    # Is string composed of all ASCII (no UTF) characters? (note str().isascii() introduced in python 3.7)
+    s = str(s)  # convert list, tuple, None, etc. to str
+    return len(s.encode().decode('ascii', 'ignore')) == len(s)
+
 
 def is_chinese(s='人工智能'):
     # Is string composed of any Chinese characters?