irc/py-limnoria: Mark as not compatible with Python 3.10+

Since Python 3.10 the method "collections.MutableMapping" has been
renamed to "collections.abc.MutableMapping" which leads to run time
errors:

[..]

class CacheDict(collections.MutableMapping):

AttributeError: module 'collections' has no attribute 'MutableMapping'
[..]

PR: 259979
Reviewed by: koobs (maintainer)
Approved by: portmgr (blanket)
MFH: 2021Q4