python 3.10 support

[arekm]

It's now in collections.abc as per https://docs.python.org/3/library/collections.abc.html#collections.abc.MutableMapping

    from pyxmpp2.message import Message
  File "/usr/share/python3.10/site-packages/pyxmpp2/message.py", line 29, in <module>
    from .stanza import Stanza
  File "/usr/share/python3.10/site-packages/pyxmpp2/stanza.py", line 34, in <module>
    from .stanzapayload import XMLPayload, payload_factory
  File "/usr/share/python3.10/site-packages/pyxmpp2/stanzapayload.py", line 24, in <module>
    from .interfaces import StanzaPayload
  File "/usr/share/python3.10/site-packages/pyxmpp2/interfaces.py", line 43, in <module>
    from .mainloop.interfaces import Event, QUIT, EventHandler, event_handler
  File "/usr/share/python3.10/site-packages/pyxmpp2/mainloop/__init__.py", line 27, in <module>
    from ..settings import XMPPSettings
  File "/usr/share/python3.10/site-packages/pyxmpp2/settings.py", line 43, in <module>
    from collections import MutableMapping
ImportError: cannot import name 'MutableMapping' from 'collections' (/usr/lib64/python3.10/collections/__init__.py)

Same here:

    from pyxmpp2.client import Client
  File "/usr/share/python3.10/site-packages/pyxmpp2/client.py", line 68, in <module>
    from .roster import RosterClient
  File "/usr/share/python3.10/site-packages/pyxmpp2/roster.py", line 43, in <module>
    from collections import Sequence, Mapping