Remove these private functions from the public C API:

* _PyRun_AnyFileObject()
* _PyRun_InteractiveLoopObject()
* _PyRun_SimpleFileObject()
* _Py_SourceAsString()

Move them to the internal C API: add a new pycore_pythonrun.h header
file. No longer export these functions.

Remove _PyOS_ReadlineTState variable from the public C API.
The symbol is still exported for the readline shared extension.

Add unit test on Py_MEMBER_SIZE() and some other macros.

The inclusion of PyOS_ReadlineFunctionPointer in python3dll.c was a mistake.

According to PEP 384:
> functions expecting FILE* are not part of the ABI, to avoid depending
> on a specific version of the Microsoft C runtime DLL on Windows.

https://bugs.python.org/issue43868

Rename Include/symtable.h to to Include/internal/pycore_symtable.h,
don't export symbols anymore (replace PyAPI_FUNC and PyAPI_DATA with
extern) and rename functions:

* PyST_GetScope() to _PyST_GetScope()
* PySymtable_BuildObject() to _PySymtable_Build()
* PySymtable_Free() to _PySymtable_Free()

Remove PySymtable_Build(), Py_SymtableString() and
Py_SymtableStringObject() functions.

The Py_SymtableString() function was part the stable ABI by mistake
but it could not be used, since the symtable.h header file was
excluded from the limited C API.

The Python symtable module remains available and is unchanged.

pymain_run_file() no longer encodes the filename: pass the filename
as an object to the new _PyRun_AnyFileObject() function.

Add new private functions:

* _PyRun_AnyFileObject()
* _PyRun_InteractiveLoopObject()
* _Py_FdIsInteractive()

pymain_run_startup() now pass the filename as a Python object to
_PyRun_SimpleFileObject().

Py_CompileString() is now always declared as a function by
Include/pythonrun.h. It is overriden with a macro in
Include/cpython/pythonrun.h.