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.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index f012b51..309f229 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -53,6 +53,7 @@
  */
 
 #include "Python.h"
+#include "pycore_dtoa.h"
 #include "_math.h"
 
 #include "clinic/mathmodule.c.h"