pyosys: tox for non-wheel builds, update instructions

- add `tox` to list of dependencies: saves builder(s) from manually having to manage a venv for python build dependencies
  - when building wheels, pip automatically creates the environment with those dependencies, so no need for tox
  - when running simply `make ENABLE_PYOSYS=1`, this is not the case. people attempting to `pip3 install --upgrade pybind11 cxxheaderparser` to add it to their system packages will be met with a scare message about "breaking system packages"
  - tox is available from all major package managers all the way back to ubuntu 20.04 and resolves this issue
- update installation instructions to drop boost and add tox instead
- update ci scripts to use `macos-15[-intel]` (`macos-13` sunset in early december)

docs/source/using_yosys/pyosys.rst

@@ -28,6 +28,14 @@ methods:
 
    ``yosys -y ./my_pyosys_script.py``
 
+   Do note this requires some build-time dependencies to be available to Python,
+   namely, ``pybind11`` and ``cxxheaderparser``. By default, the required
+   ``tox`` package will be used to create an ephemeral environment with the
+   correct versions of the tools installed.
+
+   You can force use of your current Python environment by passing the Makefile
+   flag ``PYOSYS_USE_TOX=0``.
+
 2. Installing the Pyosys wheels
 
    On macOS and GNU/Linux you can install pre-built wheels of Yosys using