cc @encukou

Functions documented at:

• https://docs.python.org/dev/c-api/marshal.html#c.PyMarshal_WriteLongToFile
• https://docs.python.org/dev/c-api/marshal.html#c.PyMarshal_WriteObjectToFile

Note: even if PyMarshal_ReadObjectFromString() and PyMarshal_WriteObjectToString() are listed in Misc/stable_abi.txt, it's not mentioned in the documentation that they are part of the stable ABI. I don't know if it is a deliberate choice or not. Misc/stable_abi.txt contains a comment:

# The following are defined in private headers, but historically
# they were exported as part of the stable ABI.
function PyMarshal_ReadObjectFromString
    added 3.2
    abi_only
function PyMarshal_WriteObjectToString
    added 3.2
    abi_only

If PyMarshal_ReadObjectFromString() is only part of the stable API but not part of the limited C API, the current mashal.h is wrong, no?

I'm trying to remove #include <stdio.h> from Python.h if Py_LIMITED_API is defined. marshal.h prevented me to do that, so I merged my change.

See: https://bugs.python.org/issue45434

The PyMarshal_*String are now defined in a private header, so they're effectively not part of the limited API.
But if they were in public headers in 3.3, the symbols need to remain part of the stable ABI.

Since marshal.h is not included in <Python.h> I don't think it necessarily needs the Py_LIMITED_API guard.
But it would be nice to be explicit and add them – to the whole file.