Support for free-threaded Python

Nobody has claimed this yet.

Installing from source on Python 3.13 (M1 Mac) works fine:

```
$ python -VV
Python 3.13.0 (main, Oct 16 2024, 08:05:40) [Clang 18.1.8 ]
$ uv pip install "httptools==0.6.4" --no-binary :all:
Resolved 1 package in 1.31s
Built httptools==0.6.4
Prepared 1 package in 5.47s
Installed 1 package in 8ms
+ httptools==0.6.4
```

But in free-threaded Python:

```
$ python -VV
Python 3.13.0 experimental free-threading build (main, Oct 16 2024, 08:24:33) [Clang 18.1.8 ]
$ uv pip install "httptools==0.6.4" --no-binary :all:
Resolved 1 package in 1.14s
error: Failed to prepare distributions
Caused by: Failed to download and build `httptools==0.6.4`
Caused by: Build backend failed to build wheel through `build_wheel` (exit status: 1)

[stdout]
running bdist_wheel
running build
running build_py
copying httptools/_version.py -> build/lib.macosx-11.0-arm64-cpython-313t/httptools
copying httptools/__init__.py -> build/lib.macosx-11.0-arm64-cpython-313t/httptools
copying httptools/parser/__init__.py -> build/lib.macosx-11.0-arm64-cpython-313t/httptools/parser
copying httptools/parser/errors.py -> build/lib.macosx-11.0-arm64-cpython-313t/httptools/parser
running egg_info
writing httptools.egg-info/PKG-INFO
writing dependency_links to httptools.egg-info/dependency_links.txt
writing requirements to httptools.egg-info/requires.txt
writing top-level names to httptools.egg-info/top_level.txt
reading manifest file 'httptools.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
adding license file 'LICENSE'
writing manifest file 'httptools.egg-info/SOURCES.txt'
copying httptools/parser/cparser.pxd -> build/lib.macosx-11.0-arm64-cpython-313t/httptools/parser
copying httptools/parser/parser.pyx -> build/lib.macosx-11.0-arm64-cpython-313t/httptools/parser
copying httptools/parser/python.pxd -> build/lib.macosx-11.0-arm64-cpython-313t/httptools/parser
copying httptools/parser/url_cparser.pxd -> build/lib.macosx-11.0-arm64-cpython-313t/httptools/parser
copying httptools/parser/url_parser.pyx -> build/lib.macosx-11.0-arm64-cpython-313t/httptools/parser
running build_ext
building 'httptools.parser.parser' extension
clang -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall -arch arm64 -mmacosx-version-min=11.0 -Wno-nullability-completeness -Wno-expansion-to-defined -Wno-undef-prefix -fPIC -Werror=unguarded-availability-new -I/Users/juan_cano/.cache/uv/sdists-v5/index/d2e01f6a682f0b5c/httptools/0.6.4/uI2NjLIM8YFjwSjCP_QiN/httptools-0.6.4.tar.gz/vendor/llhttp/include -I/Users/juan_cano/.cache/uv/sdists-v5/index/d2e01f6a682f0b5c/httptools/0.6.4/uI2NjLIM8YFjwSjCP_QiN/httptools-0.6.4.tar.gz/vendor/llhttp/src -I/Users/juan_cano/.cache/uv/builds-v0/.tmpAVsfvs/include -I/Users/juan_cano/.local/share/uv/python/cpython-3.13.0+freethreaded-macos-aarch64-none/include/python3.13t -c httptools/parser/parser.c -o build/temp.macosx-11.0-arm64-cpython-313t/httptools/parser/parser.o -O2

[stderr]
/Users/juan_cano/.cache/uv/builds-v0/.tmpAVsfvs/lib/python3.13t/site-packages/setuptools/_distutils/dist.py:261: UserWarning: Unknown distribution option: 'test_suite'
warnings.warn(msg)
httptools/parser/parser.c:2189:80: error: unknown type name '__pyx_vectorcallfunc'; did you mean 'vectorcallfunc'?
2189 | static CYTHON_INLINE PyObject *__Pyx_PyVectorcall_FastCallDict(PyObject *func, __pyx_vectorcallfunc vc, PyObject *const *args, size_t nargs, PyObject *kw);
| ^~~~~~~~~~~~~~~~~~~~
| vectorcallfunc
/Users/juan_cano/.local/share/uv/python/cpython-3.13.0+freethreaded-macos-aarch64-none/include/python3.13t/object.h:495:21: note: 'vectorcallfunc' declared here
495 | typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args,
| ^
httptools/parser/parser.c:14183:69: error: unknown type name '__pyx_vectorcallfunc'; did you mean 'vectorcallfunc'?
14183 | static PyObject *__Pyx_PyVectorcall_FastCallDict_kw(PyObject *func, __pyx_vectorcallfunc vc, PyObject *const *args, size_t nargs, PyObject *kw)
| ^~~~~~~~~~~~~~~~~~~~
| vectorcallfunc
/Users/juan_cano/.local/share/uv/python/cpython-3.13.0+freethreaded-macos-aarch64-none/include/python3.13t/object.h:495:21: note: 'vectorcallfunc' declared here
495 | typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args,
| ^
httptools/parser/parser.c:14228:80: error: unknown type name '__pyx_vectorcallfunc'; did you mean 'vectorcallfunc'?
14228 | static CYTHON_INLINE PyObject *__Pyx_PyVectorcall_FastCallDict(PyObject *func, __pyx_vectorcallfunc vc, PyObject *const *args, size_t nargs, PyObject *kw)
| ^~~~~~~~~~~~~~~~~~~~
| vectorcallfunc
/Users/juan_cano/.local/share/uv/python/cpython-3.13.0+freethreaded-macos-aarch64-none/include/python3.13t/object.h:495:21: note: 'vectorcallfunc' declared here
495 | typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args,
| ^
httptools/parser/parser.c:14917:6: error: unknown type name '__pyx_vectorcallfunc'; did you mean 'vectorcallfunc'?
14917 | __pyx_vectorcallfunc vc = __Pyx_CyFunction_func_vectorcall(cyfunc);
| ^~~~~~~~~~~~~~~~~~~~
| vectorcallfunc
/Users/juan_cano/.local/share/uv/python/cpython-3.13.0+freethreaded-macos-aarch64-none/include/python3.13t/object.h:495:21: note: 'vectorcallfunc' declared here
495 | typedef PyObject *(*vectorcallfunc)(PyObject *callable, PyObject *const *args,
| ^
4 errors generated.
error: command '/usr/bin/clang' failed with exit code 1
```

Start with httptools/parser/parser.pyx, httptools/parser/url_parser.pyx, and the generated httptools/parser/parser.c, then inspect how the extension is generated and built for Python 3.13t. Reproduce the source installation command with the free-threaded Python build; done means httptools builds and installs successfully without the reported compilation errors.

Written by the indexing model from the issue text.