bpo-35081: Move dtoa.h header to the internal C API (GH-18489)

Move the dtoa.h header file to the internal C API as pycore_dtoa.h:
it only contains private functions (prefixed by "_Py").

The math and cmath modules must now be compiled with the
Py_BUILD_CORE macro defined.

Python/dtoa.c

@@ -115,6 +115,7 @@
 /* Linking of Python's #defines to Gay's #defines starts here. */
 
 #include "Python.h"
+#include "pycore_dtoa.h"
 
 /* if PY_NO_SHORT_FLOAT_REPR is defined, then don't even try to compile
    the following code */

Python/pystrtod.c

@@ -1,6 +1,7 @@
 /* -*- Mode: C; c-file-style: "python" -*- */
 
 #include <Python.h>
+#include "pycore_dtoa.h"
 #include <locale.h>
 
 /* Case-insensitive string match used for nan and inf detection; t should be