Issue 40415: _asyncio extensions crashes if initialized multiple times in same process

Most of CPython's extensions can be initialized and freed multiple times in the same process. However, _asyncio crashes on at least CPython 3.8.2 when this is done.

STR:

1. Create a new Python interpreter
2. Have it import _asyncio
3. Finalize that interpreter.
4. Create a new Python interpreter
5. Have it import _asyncio

There are probably STR in pure Python by forcing _imp.create_dynamic() to run multiple times after the module is unloaded.

The crash occurs due to unchecked NULL access in `Py_INCREF(all_tasks);` in `PyInit__asyncio()`.

I think the underlying problem is module_init() is short-circuiting because `module_initialized` is set. And `module_initialized` is set on subsequent module loads because `module_free()` isn't clearing it.

Oh, I just went to patch this and it is a dupe of 40294, which has already been fixed and backported.

stage: resolved