bpo-35713: Rework Python initialization

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• Remove PyByteArray_Init() and PyByteArray_Fini() functions, they
  did nothing since Python 2.7.4 and Python 3.2.0.
• _PyExc_Init() now returns an error on failure rather than calling
  Py_FatalError(). Move macros inside _PyExc_Init() and undefine them
  when done. Rewrite macros to make them look more like statement:
  add ";" when using them, add "do { ... } while (0)".
• _PyUnicode_Init() now returns a _PyInitError error rather than call
  Py_FatalError().
• Move stdin check from _PySys_BeginInit() to init_sys_streams().
• _Py_ReadyTypes() now returns a _PyInitError error rather than
  calling Py_FatalError().
• Move "_PyXXX_Init()" and "_PyXXX_Fini()" declarations from
  Include/cpython/pylifecycle.h to
  Include/internal/pycore_pylifecycle.h. Replace
  "PyAPI_FUNC(TYPE)" with "extern TYPE".

https://bugs.python.org/issue35713

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@scoder, @ericsnowcurrently: Do you think that it's ok to remove "PyXXX_Init()" from the C API? I don't see why anyone would call them explicitly?

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@scoder, @ericsnowcurrently: Do you think that it's ok to remove "PyXXX_Init()" from the C API? I don't see why anyone would call them explicitly?

Ignore my questions. In fact, these functions are prefixed by "_Py": they are private, and we don't provide any stability warranty for the private API.

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I modified my PR to document properly the removal of PyByteArray_Init() and PyByteArray_Fini() functions: documented in the Porting section of What's New in Python 3.8 and in a NEWS entry.