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)

.github/workflows/test-compile.yml

@@ -45,7 +45,7 @@ jobs:
           - 'gcc-14'
         include:
           # macOS x86
-          - os: macos-13
+          - os: macos-15-intel
             compiler: 'clang-19'
           # macOS arm
           - os: macos-latest

.github/workflows/wheels.yml

@@ -27,13 +27,13 @@ jobs:
             {
               name: "macOS 13",
               family: "macos",
-              runner: "macos-13",
+              runner: "macos-15-intel",
               archs: "x86_64",
             },
             {
               name: "macOS 14",
               family: "macos",
-              runner: "macos-14",
+              runner: "macos-15",
               archs: "arm64",
             },
             ## Windows is disabled because of an issue with compiling FFI as