Previously, the result could have been an instance of a subclass of int.

Also revert bpo-26202 and make attributes start, stop and step of the range
object having exact type int.

Add private function _PyNumber_Index() which preserves the old behavior
of PyNumber_Index() for performance to use it in the conversion functions
like PyLong_AsLong().

Only __index__ should be used to make integer conversions lossless.

… now that we allow subtypes of PyCFunction.

Also add PyCMethod_CheckExact() and PyCMethod_Check() for checks against the PyCMethod subtype.

* Replace id->object with _PyUnicode_FromId(&id)
* Use _Py_static_string_init(str) macro to initialize statically
  name_op in typeobject.c.

If only offsetof() is needed: include stddef.h instead.

When structmember.h is used, add a comment explaining that
PyMemberDef is used.

Always declare PyIndex_Check() as an opaque function to hide
implementation details: remove PyIndex_Check() macro. The macro
accessed directly the PyTypeObject.tp_as_number member.

Add _PyIndex_Check() function to the internal C API: fast inlined
verson of PyIndex_Check().

Add Include/internal/pycore_abstract.h header file.

Replace PyIndex_Check() with _PyIndex_Check() in C files of Objects
and Python subdirectories.

Convert PyObject_CheckBuffer() macro to a function to hide
implementation details: the macro accessed directly the
PyTypeObject.tp_as_buffer member.

This implements things like `list[int]`,
which returns an object of type `types.GenericAlias`.
This object mostly acts as a proxy for `list`,
but has attributes `__origin__` and `__args__`
that allow recovering the parts (with values `list` and `(int,)`.

There is also an approximate notion of type variables;
e.g. `list[T]` has a `__parameters__` attribute equal to `(T,)`.
Type variables are objects of type `typing.TypeVar`.