Fix Python 3.10 support

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tests/conftest.py:11: in <module>
    from circuits import BaseComponent, Debugger, Manager, handler
circuits/__init__.py:20: in <module>
    from .core import (
circuits/core/__init__.py:5: in <module>
    from .bridge import Bridge, ipc
circuits/core/bridge.py:12: in <module>
    from .components import BaseComponent
circuits/core/components.py:4: in <module>
    from collections import Callable
E   ImportError: cannot import name 'Callable' from 'collections' (/opt/hostedtoolcache/Python/3.10.0-rc.1/x64/lib/python3.10/collections/__init__.py)

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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'

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Tests are working again.