import 'escape' from html, not from cgi

Under python3.8 'from cgi import escape' will fail.  Import from html instead.

Author: MiddleFork

nbviewer/providers/base.py

@@ -12,7 +12,7 @@
 import statsd
 import asyncio
 
-from cgi import escape
+from html import escape
 from contextlib import contextmanager
 from datetime import datetime