gh-101819: Prepare to modernize the _io extension (#104178)

* Add references to static types to _PyIO_State:

  * PyBufferedIOBase_Type
  * PyBytesIOBuffer_Type
  * PyIncrementalNewlineDecoder_Type
  * PyRawIOBase_Type
  * PyTextIOBase_Type

* Add the defining class to methods:

  * _io.BytesIO.getbuffer()
  * _io.FileIO.close()

* Add get_io_state_by_cls() function.
* Add state parameter to _textiowrapper_decode()
* _io_TextIOWrapper___init__() now sets self->state before calling
  _textiowrapper_set_decoder().

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>