Add fileobject.pxd

The comments are taken from the C API documentation
(https://docs.python.org/3.9/c-api/file.html).

PyObject_AsFileDescriptor() is already present in object.pxd.

Cython/Includes/cpython/fileobject.pxd

+cdef extern from "Python.h":
+
+    ###########################################################################
+    # File Objects
+    ###########################################################################
+
+    object PyFile_FromFd(int fd, const char *name, const char *mode, int buffering, const char *encoding, const char *errors, const char *newline, int closefd)
+    # Return value: New reference.
+    # Create a Python file object from the file descriptor of an already
+    # opened file fd. The arguments name, encoding, errors and newline can be
+    # NULL to use the defaults; buffering can be -1 to use the default. name
+    # is ignored and kept for backward compatibility. Return NULL on failure.
+    # For a more comprehensive description of the arguments, please refer to
+    # the io.open() function documentation.
+
+    # Warning: Since Python streams have their own buffering layer, mixing
+    # them with OS-level file descriptors can produce various issues (such as
+    # unexpected ordering of data).
+
+    # Changed in version 3.2: Ignore name attribute.
+
+    object PyFile_GetLine(object p, int n)
+    # Return value: New reference.
+    # Equivalent to p.readline([n]), this function reads one line from the
+    # object p. p may be a file object or any object with a readline()
+    # method. If n is 0, exactly one line is read, regardless of the length of
+    # the line. If n is greater than 0, no more than n bytes will be read from
+    # the file; a partial line can be returned. In both cases, an empty string
+    # is returned if the end of the file is reached immediately. If n is less
+    # than 0, however, one line is read regardless of length, but EOFError is
+    # raised if the end of the file is reached immediately.
+
+    int PyFile_WriteObject(object obj, object p, int flags) except? -1
+    # Write object obj to file object p. The only supported flag for flags
+    # is Py_PRINT_RAW; if given, the str() of the object is written instead of
+    # the repr(). Return 0 on success or -1 on failure; the appropriate
+    # exception will be set.
+
+    int PyFile_WriteString(const char *s, object p) except? -1
+    # Write string s to file object p. Return 0 on success or -1 on failure;
+    # the appropriate exception will be set.

tests/run/cython_includes.pyx

@@ -18,6 +18,7 @@ cimport cpython.complex
 cimport cpython.datetime
 cimport cpython.dict
 cimport cpython.exc
+cimport cpython.fileobject
 cimport cpython.float
 cimport cpython.function
 cimport cpython.genobject