bpo-35081: Move accu.h to Include/internal/pycore_accu.h

[vstinner]

The accu.h header is no longer part of the Python C API: it has been
moved to the "internal" headers which are restricted to Python
itself.

Replace #include "accu.h" with #include "pycore_accu.h".

https://bugs.python.org/issue35081

[vstinner]

I tried to enforce to require Py_BUILD_CORE in pycore_accu.h to be defined using:

#ifndef Py_BUILD_CORE
#  error "Py_BUILD_CORE must be defined to include this header"
#endif

But the compilation of the _json module failed, because it isn't compiled with Py_BUILD_CORE. Moreover, _json.c contains:

/* Core extension modules are built-in on some platforms (e.g. Windows). */
#ifdef Py_BUILD_CORE
#define Py_BUILD_CORE_BUILTIN
#undef Py_BUILD_CORE
#endif