Add support for Python 3.13rc1 and drop support for Python 3.7. (#348)

* Try building on Python 3.13b1.

* Use 3.13.0-beta.1.

* Update all.yml

* Update all.yml

* Enable building wheels for pre-release Pythons for now.

* Remove libexecinfo.

* Vendor execinfo.h (as the code is not used but required by JUCE).

* Add empty function bodies for execinfo.

* Add empty function bodies for execinfo.

* Try nogil builds.

* Upgrade PyBind11 to 2.13.0.

* Update all.yml

* Enable CIBW_FREE_THREADED_SUPPORT.

* Disable MUSL builds for free-threaded Python.

* Disable MUSL builds for 3.13 beta.

* Try only 3.13 on Linux for now.

* Update to 3.13.0-rc.1.

* Update all.yml

* Update pyproject.toml

* Update pyproject.toml

* Update list of supported Python versions.

* Stop building for Python 3.6.

* Use NumPy 2.10 for Python 3.13.

* Fix numpy dependencies in test-requirements.