Support building free-threaded wheels

[ngoldbaum]

While working on crate-py/rpds#101 I discovered that maturin can't build free-threaded wheels:

± maturin build --target aarch64-apple-darwin --release --out dist --interpreter 3.9 3.10 3.11 3.12 3.13 3.13t pypy3.9 pypy3.10
📦 Including license file "/Users/goldbaum/Documents/rpds-py/LICENSE"
🔗 Found pyo3 bindings
⚠️  Warning: skipped unavailable python interpreter 'pypy3.10' from pyenv
💥 maturin failed
  Caused by: Invalid python interpreter minor version '13t', expect a digit
  Caused by: invalid digit found in string

I'm not sure if there are other issues besides that there isn't a UI to do this, I don't know much about how maturin works.

It might be helpful to look at what @henryiii and @mayeut did to get cibuildwheel working with the free-threaded build: pypa/cibuildwheel#1831

[davidhewitt]

This is already underway in #2310 😁

[davidhewitt]

@messense is there anything remaining to do before this can be released? I'm happy to offer help if there is, would love to see a free-threaded compatible maturin so that we can start shipping 🛥️

[messense]

Opened #2322

[davidhewitt]

🙏 amazing, thanks!