fix: allow pip install when not the repo root

[rjwalters]

Allow pip install when package is not at repo root

Problem

Currently, PufferLib can only be installed properly when it's located at the root of a repository. This makes it difficult to vendor the library as a subdirectory (e.g., deps/pufferlib) or use it as a submodule, especially with editable installs.

Solution

This PR makes PufferLib installable regardless of its location in the directory structure by:

1. Scoping find_packages() to pufferlib explicitly
2. Using package_dir={"": "pufferlib"} to specify the package root
3. Updating Cython paths to be relative to the project root
4. Adding pyproject.toml fields to support modern build tools and PEP 660

Benefits

• Fully backward-compatible: Existing installations and builds work exactly as before
• Improved flexibility: Can now be vendored in subdirectories or used as a submodule
• Better editable installs: -e installs work correctly regardless of package location
• Modern standards: Adds proper PEP 660 support via pyproject.toml configuration

These changes are minimal and focused solely on improving the installation process without affecting functionality or existing workflows.

[rjwalters]

This was an attempt to get pufferlib to install in editable mode within a a subfolder (/deps/pufferlib)

pip install -e .

I could not get it to work but it seems to work in normal mode:

pip install .

the error was

error: could not create 'deps/pufferlib/pufferlib/extensions.cpython-311-darwin.so': No such file or directory