bpo-43244: Remove symtable.h header file (GH-24910)

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.