install: Migrate from setup.py to pyproject.toml.

[rht]

• I use Hatch instead of Setuptools because Setuptools data-files doesn't
  support recursive file includes.

Fixes: N/A

How did you test this PR?

• Running pytest tests
• Running my own script to automate uploading a file to a given topic in a channel
• Ensured that the target data files, share/zulip has the same content as when installed in current main

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[rht]

This is superseded by readme = "README.md" in the pyproject.toml version. I checked the resulting METADATA file in the package info, and it showed the entire README.md in it.

[rht]

I confirmed that the scripts all exist in the installed virtualenv bin directory.