You can use AnyMsg to subscribe to the topic, then use this to acquire information about the topic which can be used to construct a publisher and a new message which can be published. For an example, see below.

import rospy
import genpy
from rospy.msg import AnyMsg

rospy.init_node("test_node")
publisher = None
msg_class = None


# callback that we'll use to setup the publisher once we've gotten a message
def setup_publisher(msg):
    global msg_class
    global publisher

    topic_type = msg._connection_header["type"]
    msg_class = genpy.message.get_message_class(topic_type)
    publisher = rospy.Publisher("test_topic", msg_class, queue_size=10)

# subscribe and wait for a message to come in
check_rate = rospy.Rate(10)
sub = rospy.Subscriber("test_topic", AnyMsg, setup_publisher)
while publisher is None and not rospy.is_shutdown():
    check_rate.sleep()
sub.unregister() # all done with the subscriber now

assert publisher is not None
while not rospy.is_shutdown():
    # now that we have a valid publisher we can create a new message.
    # we could use genpy like in the callback, or we can just use the
    # publisher:
    new_msg = publisher.data_class()

    # even if we don't know what type it is beforehand,
    # we can find out what information can be populated
    # thanks to the 'slots' attribute provided by messages
    if "data" in new_msg.__slots__:
        if type(new_msg.data) == int:
            new_msg.data = 42
        elif type(new_msg.data) == str:
            new_msg.data = "Hello World!"
        elif type(new_msg.data) == float:
            new_msg.data = 3.14159

    # we can also make use of the setattr/gettattr
    # functions in a slightly more traditionally-python way
    for slot in dir(new_msg):
        if slot.startswith("_"):
            # ignore private attributes
            continue
        if type(getattr(new_msg, slot)) == int:
            # set any integer to 42
            setattr(new_msg, slot, 42)

    # now we can publish the message
    publisher.publish(new_msg)