Add cpython.marshal declarations (GH-3468)

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/marshal.html).

Cython/Includes/cpython/marshal.pxd

+from libc.stdio cimport FILE
+
+cdef extern from "Python.h":
+
+    ###########################################################################
+    # Data marshalling support
+    ###########################################################################
+
+    const int Py_MARSHAL_VERSION
+
+    void PyMarshal_WriteLongToFile(long value, FILE *file, int version)
+    # Marshal a long integer, value, to file. This will only write the
+    # least-significant 32 bits of value, regardless of the size of the native
+    # long type. version indicates the file format.
+
+    void PyMarshal_WriteObjectToFile(object value, FILE *file, int version)
+    # Marshal a Python object, value, to file. version indicates the file
+    # format.
+
+    bytes PyMarshal_WriteObjectToString(object value, int version)
+    # Return value: New reference.
+    # Return a bytes object containing the marshalled representation of value.
+    # version indicates the file format.
+
+    long PyMarshal_ReadLongFromFile(FILE *file) except? -1
+    # Return a C long from the data stream in a FILE* opened for reading. Only
+    # a 32-bit value can be read in using this function, regardless of the
+    # native size of long.
+
+    # On error, sets the appropriate exception (EOFError) and returns -1.
+
+    int PyMarshal_ReadShortFromFile(FILE *file) except? -1
+    # Return a C short from the data stream in a FILE* opened for reading. Only
+    # a 16-bit value can be read in using this function, regardless of the
+    # native size of short.
+
+    # On error, sets the appropriate exception (EOFError) and returns -1.
+
+    object PyMarshal_ReadObjectFromFile(FILE *file)
+    # Return value: New reference.
+    # Return a Python object from the data stream in a FILE* opened for
+    # reading.
+    
+    # On error, sets the appropriate exception (EOFError, ValueError or
+    # TypeError) and returns NULL.
+
+    object PyMarshal_ReadLastObjectFromFile(FILE *file)
+    # Return value: New reference.
+    # Return a Python object from the data stream in a FILE* opened for
+    # reading. Unlike PyMarshal_ReadObjectFromFile(), this function assumes
+    # that no further objects will be read from the file, allowing it to
+    # aggressively load file data into memory so that the de-serialization can
+    # operate from data in memory, rather than reading a byte at a time from the
+    # file. Only use these variant if you are certain that you won’t be reading
+    # anything else from the file.
+
+    # On error, sets the appropriate exception (EOFError, ValueError or
+    # TypeError) and returns NULL.
+
+    object PyMarshal_ReadObjectFromString(const char *data, Py_ssize_t len)
+    # Return value: New reference.
+    # Return a Python object from the data stream in a byte buffer containing
+    # len bytes pointed to by data.
+
+    # On error, sets the appropriate exception (EOFError, ValueError or
+    # TypeError) and returns NULL.

tests/run/cython_includes.pyx

@@ -28,6 +28,7 @@ cimport cpython.list
 cimport cpython.long
 cimport cpython.longintrepr
 cimport cpython.mapping
+cimport cpython.marshal
 cimport cpython.mem
 cimport cpython.memoryview
 cimport cpython.method