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

[twavv]

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.

For now, you can run help(str) or print(str.__doc__), but the output is not quite as helpful.