(cherry picked from commit d382bde220b4c07cce2b924ffeb7525ea1a969f4)

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>

Add the Py_Is(x, y) function to test if the 'x' object is the 'y'
object, the same as "x is y" in Python. Add also the Py_IsNone(),
Py_IsTrue(), Py_IsFalse() functions to test if an object is,
respectively, the None singleton, the True singleton or the False
singleton.

This change partically reverts
commit ad3252bad905d41635bcbb4b76db30d570cf0087
and the commit fe2978b3b940fe2478335e3a2ca5ad22338cdf9c.

Many third party C extension modules rely on the ability of using
Py_TYPE() to set an object type: "Py_TYPE(obj) = type;" or to set an
object type using: "Py_SIZE(obj) = size;".

Add "borrowed reference" and "strong reference" to the documentation
glossary.

Enhance also Py_INCREF() and Py_NewRef() documentation.

For example, fix the following Sphinx 3 errors:

Doc/c-api/buffer.rst:102: WARNING: Error in declarator or parameters
Invalid C declaration: Expected identifier in nested name. [error at 5]
  void \*obj
  -----^

Doc/c-api/arg.rst:130: WARNING: Unparseable C cross-reference: 'PyObject*'
Invalid C declaration: Expected end of definition. [error at 8]
  PyObject*
  --------^

The modified documentation is compatible with Sphinx 2 and Sphinx 3.

Convert Py_REFCNT() and Py_SIZE() macros to static inline functions.
They cannot be used as l-value anymore: use Py_SET_REFCNT() and
Py_SET_SIZE() to set an object reference count and size.

Replace &Py_SIZE(self) with &((PyVarObject*)self)->ob_size
in arraymodule.c.

This change is backward incompatible on purpose, to prepare the C API
for an opaque PyObject structure.