Add support for GIL-free Python runtimes

This adds experimental support for running pyosmium in [free-threading mode](https://py-free-threading.github.io/). There are no changes to the code required as pyosmium doesn't keep global state and is already reentrant. Objects created by the library are **not** thread-safe for writing. This is by design to keep overhead low for the usual non-threaded case. It is the user's responsibility to guard write accesses with locks as required. This behaviour is now documented. Tests needed some restructuring to make them executable in parallel. You can now run the tests using [pytest-run-parallel](https://github.com/Quansight-Labs/pytest-run-parallel). The CI will now build for 3.13t and 3.14t and test those build in parallel mode. The PR also enables 3.14 for the CI. Currently the test requiring shapely is disabled because there are no binary wheels for shapely yet and compiling on the fly is a major pain.