With Cython 3 the default for functions, that return void and have no
explicit `except` specification, was switched from `noexcept` to `except *`
even for functions that are nogil. As the result, when Cython 3
compiles e.g. golang/_golang.pyx, it complains with

        void _taskgo_pyexc(void (*f)(void *) nogil, void *arg)      except +topyexc:
            _taskgo(f, arg)
                    ^
    ------------------------------------------------------------

    golang/_golang.pyx:596:16: Cannot assign type 'void (*)(void *) except * nogil' to 'void (*)(void *) noexcept nogil'. Exception values are incompatible. Suggest adding 'noexcept' to the type of 'f'.

In pygolang there are many nogil functions and the perception of those
are that they are unrelated to python world unless explicitly programmed
via e.g. `with gil` sections inside. So a `noexcept` specification on
such nogil functions could be a bit misleading to the reader suggestion
that this noexcept is about e.g. C++ part or panic.

-> Avoid this confusion by activating "legacy" mode of having "no py
except" by default if that is not specified.

It works on both Cython 3 and Cython 0.29.x because, added directive is
simply ignored by Cython 0.29.x and the builtin behaviour is already ok.

/reviewed-by @jerome
/reviewed-on !37