Fix gen_is_coroutine for Python 3.13

[cdce8p]

The _PyInterpreterFrame struct was changed in python/cpython#105727 to store the code object in f_executable instead of f_code.

Fixes

  /home/runner/work/mypy/mypy/mypyc/lib-rt/pythonsupport.h: In function ‘_PyGen_GetCode’: (diff)
  /home/runner/work/mypy/mypy/mypyc/lib-rt/pythonsupport.h:403:17: error: ‘_PyInterpreterFrame’ has no member named ‘f_code’ (diff)
    403 |     return frame->f_code; (diff)
        |                 ^~ (diff)

[hauntsaninja]

I didn't quite follow the point of the CPython changes, is this field actually used for non-code objects now?

[cdce8p]

I didn't quite follow the point of the CPython changes, is this field actually used for non-code objects now?

I believe the idea is to reuse this field for non-code objects / not needing to create fake code object to be able to use it.
Likely doesn't matter for us, at least not at this point.