setup: set PYO3_PYTHON to ensure PyO3 uses the right Python
We're currently setting PYTHON_SYS_EXECUTABLE in setup.py to ensure that rust-cpython uses the correct Python installation. In the case of PyO3, we need to set PYO3_PYTHON instead.
We're currently setting PYTHON_SYS_EXECUTABLE in setup.py to ensure that rust-cpython uses the correct Python installation. In the case of PyO3, we need to set PYO3_PYTHON instead.