Feature: Support ?help syntax in Python kernels (Jupyter "Payloads")

In Python, you can pull up the docs for `str` by running `?str` in a code cell. This is not currently implemented in Pathbird since it uses the [(technically deprecated) Jupyter "Payloads" API](https://jupyter-client.readthedocs.io/en/stable/messaging.html#payloads-deprecated).

For now, you can run `help(str)` or `print(str.__doc__)`, but the output is not quite as helpful.