- 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)
