Make python buildable with pip

[sarahec]

This reconfigures the python extension to allow building directly using pip:

1. Adds a sqlite_vec folder to conform with the flat package layout.
2. Adds a proper __init__.py so the package can be imported.
3. Adds minimal pyproject.toml and requirements.txt for a basic setuptools build.
4. Adds a pytest file to quickly validate the extension.
5. Configured to pull the version number from git. (This can be edited to read the version file if preferred.)

Fixes #230 with the caveat that we'll need to apply sed to __init__.py at build time. I'm open to suggestions on how to do this.

P.S. This enables packaging on nixpkgs: NixOS/nixpkgs#420393