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/getting_started/installation.rst

@@ -99,8 +99,7 @@ Installing all prerequisites for Ubuntu 22.04:
 
    sudo apt-get install gperf build-essential clang lld bison flex libfl-dev \
       libreadline-dev gawk tcl-dev libffi-dev git \
-      graphviz xdot pkg-config python3 libboost-system-dev \
-      libboost-python-dev libboost-filesystem-dev zlib1g-dev
+      graphviz xdot pkg-config python3-dev tox zlib1g-dev
 
 Installing all prerequisites for macOS 13 (with Homebrew):
 
@@ -141,7 +140,7 @@ For Cygwin use the following command to install all prerequisites, or select the
    missing `strdup` function when using gcc.  It is instead recommended to use
    Windows Subsystem for Linux (WSL) and follow the instructions for Ubuntu.
 
-.. 
+..
    For MSYS2 (MINGW64):
 
    .. code:: console
@@ -215,7 +214,7 @@ Running the build system
 From the root ``yosys`` directory, call the following commands:
 
 .. code:: console
-   
+
    make
    sudo make install
 
@@ -228,7 +227,7 @@ To use a separate (out-of-tree) build directory, provide a path to the Makefile.
 
 Out-of-tree builds require a clean source tree.
 
-.. seealso:: 
+.. seealso::
 
    Refer to :doc:`/yosys_internals/extending_yosys/test_suites` for details on
    testing Yosys once compiled.

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