Fix test.support.Py_C_RECURSION_LIMIT to reflect Include/cpython/pystate.h more accurately

[brettcannon]

No description provided.

[vstinner]

The _testcapi extension should always be available when running the Python test suite. I suppose that the ImportError code path is for Python implementation others than CPython, no?

[brettcannon]

The _testcapi extension should always be available when running the Python test suite. I suppose that the ImportError code path is for Python implementation others than CPython, no?

I have no idea since I didn't add that code.

[vstinner]

@markshannon: Do you have an opinion on this change?

@brettcannon: If you want to sync the Python support code with the C header file, you may also add the Py_DEBUG case. (But I'm asking if it's worth it, since you can expect the _testcapi extension to be available in CPython.)

[brettcannon]

If we end up not bothering with this PR then I will update the comment to drop the pystate.h comment as it's misleading if it doesn't have to be kept in sync.

[brettcannon]

Switching to just clarifying the comment so it implies it doesn't need to be kept in sync with the header file.