multithreaded situations. If ``block`` is set to False, more
        connections will be created but they will not be saved once they've
        been used.

    :param block:
        If set to True, no more than ``maxsize`` connections will be used at
        a time. When no free connections are available, the call will block
        until a connection has been released. This is a useful side effect for
        particular multithreaded situations where one does not want to use more
        than maxsize connections per host to prevent flooding.

    :param headers:
        Headers to include with all requests, unless other headers are given
        explicitly.

    :param retries:
        Retry configuration to use by default with requests in this pool.

    :param _proxy:
        Parsed proxy URL, should not be used directly, instead, see
        :class:`urllib3.ProxyManager`

    :param _proxy_headers:
        A dictionary with proxy headers, should not be used directly,
        instead, see :class:`urllib3.ProxyManager`

    :param \**conn_kw:
        Additional parameters are used to create fresh :class:`urllib3.connection.HTTPConnection`,
        :class:`urllib3.connection.HTTPSConnection` instances.
    