Fix Python 3.10 support about circuits

[spaceone]

tests/web/conftest.py:8: in <module>
    from circuits.web import Server, Static
circuits/web/__init__.py:7: in <module>
    from .dispatchers import XMLRPC, Dispatcher, Static, VirtualHosts
circuits/web/dispatchers/__init__.py:8: in <module>
    from .dispatcher import Dispatcher
circuits/web/dispatchers/dispatcher.py:15: in <module>
    from circuits.web.processors import process
circuits/web/processors.py:7: in <module>
    from .parsers import MultipartParser, QueryStringParser
circuits/web/parsers/__init__.py:4: in <module>
    from .multipart import MultipartParser
circuits/web/parsers/multipart.py:61: in <module>
    from UserDict import DictMixin
E   ModuleNotFoundError: No module named 'UserDict'

[spaceone]

Tests are working again.