Get a :class:`ConnectionPool` based on the request context.

        ``request_context`` must at least contain the ``scheme`` key and its
        value must be a key in ``key_fn_by_scheme`` instance variable.

def connection_from_context(self, request_context):
        """
        Get a :class:`ConnectionPool` based on the request context.

        ``request_context`` must at least contain the ``scheme`` key and its
        value must be a key in ``key_fn_by_scheme`` instance variable.
        """
        scheme = request_context['scheme'].lower()
        pool_key_constructor = self.key_fn_by_scheme[scheme]
        pool_key = pool_key_constructor(request_context)

        return self.connection_from_pool_key(pool_key, request_context=request_context)