Configure entry point to make it easier to run `cli.py`

[JackKelly]

So pip install installs the cli. So it's available from all paths.

[JackKelly]

The maturin docs contain this relevant info, about lines to add to pyproject.toml:

Pip allows adding so called console scripts, which are shell commands that execute some function in your program. You can add console scripts in a section [project.scripts].
The keys are the script names while the values are the path to the function in the format some.module.path:class.function, where the class part is optional. The function is called with no arguments. Example:

[project.scripts]
get_42 = "my_project:DummyClass.get_42"