Add project description for PyPI

[jonbannister]

Currently PyPI's description of Notebooker is threadbare. It can be updated by fiddling with pyproject.toml. See more info here - https://packaging.python.org/en/latest/tutorials/packaging-projects/#creating-pyproject-toml

[hichem-18]

Hi I tried doing it.
this is my pull request:
hichem-18#1

[jonbannister]

Implemented with 0.7.1 by @morotti