bpo-46748: Don't import <stdbool.h> in public headers (GH-31553)

<stdbool.h> is the standard/modern way to define embedd/extends Python free to define bool, true and false, but there are existing applications that use slightly different redefinitions, which fail if the header is included.

It's OK to use stdbool outside the public headers, though.

https://bugs.python.org/issue46748