webutils: monkeypatch to make webhelpers2 work with Python 3.10

Webhelpers2 is apparently a dead project and is using collections.Sequence directly.

kallithea/lib/webutils.py

@@ -20,6 +20,7 b' thread-local "global" variables. It shou'
 imported anywhere - just like the global variables can be used everywhere.
 """
 
+import collections
 import datetime
 import json
 import logging
@@ -66,6 +67,10 b' assert truncate'
 assert wrap_paragraphs
 
 
+# work around webhelpers2 being a dead project that doesn't support Python 3.10
+collections.Sequence = collections.abc.Sequence
+
+
 #
 # General Kallithea URL handling
 #