gh-121617: Include <string.h> for Py_CLEAR() macro

[vstinner]

If the _Py_TYPEOF macro is not available, include <string.h> to get memcpy() for the Py_CLEAR() macro. The limited C API version 3.11 and newer doesn't include <string.h> anymore.

Add a Py_CLEAR() test in test_cext.

Modify also _Py_TYPEOF to use C23 typeof() if available.

• Issue: memcpy() usage without #include <string.h> in pwdmodule.c #121617

[vstinner]

If this change is merged in the main branch, I will write a backport to the 3.14 branch without the C23 change using typeof().

[vstinner]

Note: Py_SETREF() and Py_XSETREF() macros also use memcpy(), but these macros are only defined if the limited C API is not used. In this case, Python.h includes <string.h> and so we are already good (no change needed). Only Py_CLEAR() of the limited C API version 3.11 and newer is affected by the issue.

[encukou]

The documentation also needs an update. (3.14 has the list here.)

It's somewhat hard to explain, unfortunately. Is it enough to add something vague like “<string.h> (on certain compilers)”?

[vstinner]

I added the following sentence to the doc:

The <string.h> header is also included when using Limited API 3.11 or newer if Python is unable to get a typeof() implementation.

I'm not sure if it's worth it to document this corner case.