Isolate the _io extension module by moving all global variables to module state, porting static types to heap types, and implementing multi-phase init. All global variables in the _io module are static types:

• Modules/_io/bufferedio.c: PyBufferedIOBase_Type
• Modules/_io/bufferedio.c: PyBufferedRWPair_Type
• Modules/_io/bufferedio.c: PyBufferedRandom_Type
• Modules/_io/bufferedio.c: PyBufferedReader_Type
• Modules/_io/bufferedio.c: PyBufferedWriter_Type
• Modules/_io/bytesio.c: PyBytesIO_Type
• Modules/_io/bytesio.c: _PyBytesIOBuffer_Type
• Modules/_io/fileio.c: PyFileIO_Type
• Modules/_io/iobase.c: PyIOBase_Type
• Modules/_io/iobase.c: PyRawIOBase_Type
• Modules/_io/textio.c: PyIncrementalNewlineDecoder_Type
• Modules/_io/textio.c: PyTextIOBase_Type
• Modules/_io/textio.c: PyTextIOWrapper_Type
• Modules/_io/winconsoleio.c: PyWindowsConsoleIO_Type

Converting the static types to heap types involves applying PEP-687 to _io.
Adapting multi-phase init involves applying PEP-489 to _io.