Support freethreading python (#2805)
freethreaded python reintroduces abiflags since it is incompatible with
regular native modules and abi3.
Tests: None yet! We're lacking cpython 3.13 no-gil builds we can use in
ci.
My test setup:
```
PYTHON_CONFIGURE_OPTS="--enable-shared --disable-gil" pyenv install 3.13.0a5
cargo run -q -- venv -q -p python3.13 .venv3.13 --no-cache-dir && cargo run -q -- pip install -v psutil --no-cache-dir && .venv3.13/bin/python -c "import psutil"
```
Fixes #2429
