> Some compilers define false and true as macros.

Which compilers specifically? It sounds like a violation of the C
standard to do so, without stdbool.h being included.

> Using stdbool.h when it is available will ensure bool is defined as a 
> type following the correct definition, which may or may not be an enum 
> depending on the compiler.

But would that help in any way with respect to above compilers?
If they don't follow the C standard, why should they provide stdbool.h?

> Even when using gcc, stdbool.h is here to define bool in C language, so 
> why not use it ?

Because we cannot *rely* on stdbool.h being present. Therefore,
inclusion of stdbool.h must be conditional, with a fallback definition
if stdbool.h is absent, and it thus complicates the source code of
Python, with no gain whatsoever.