bpo-42591: Export missing Py_FrozenMain() symbol (GH-23730)

Export the Py_FrozenMain() function: fix a Python 3.9.0 regression.
Python 3.9 uses -fvisibility=hidden and the function was not exported
explicitly and so not exported.

Add also Py_FrozenMain to the stable ABI on Windows.
3 files changed