Switch to hatch build backend

[blink1073]

Fixes #1776

Hatch is now part of the pypa, and offers us some niceties:

• Allows us to have a more simplified build script
• Allows us to map paths for data files (called shared files)
• Avoids this error which is causing Evaluation install fails since the recent flit migration #1776
• Uses .gitignore to determine what files to include by default

[SylvainCorlay]

Hey @blink1073 thanks for modernizing the packaging of nbconvert.

Just a small nitpick: I have a slight preference for keeping the share/jupyter/nbconvert/templates directory structure in the repository (just like with kernels in https://github.com/jupyter-xeus/xeus-python), which is more explicit about where things go.

Let me know what you think.

[blink1073]

No objections from me

[maartenbreddels]

My latest 'standard' is to put everything that goes under the $prefix, in the 'prefix' subdir, I think it's an artifact from how flit wants it, but it makes it clear where things go without describing it explicitly.

├── prefix
│   └── etc
│       └── jupyter
│           ├── jupyter_notebook_config.d
│           │   └── foobar.json
│           └── jupyter_server_config.d
│               └── foobar.json

[SylvainCorlay]

That's fine by me too. I like this explicit style.