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. wcfs/internal/wcfs_test.pyx, it complains with

    def install_sigbus_trap():
        cdef sigaction_t act
        act.sa_sigaction = on_sigbus
                           ^
    ------------------------------------------------------------

    wcfs/internal/wcfs_test.pyx:192:23: Cannot assign type 'void (int, siginfo_t *, void *) except * nogil' to 'void (*)(int, siginfo_t *, void *) noexcept'. Exception values are incompatible. Suggest adding 'noexcept' to the type of 'on_sigbus'.

In wendelin.core, similarly to 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.

This patch is based on and mirrors the following pygolang patch:
kirr/pygolang@b5bb9f7e

/cc @tomo
/reviewed-by @levin.zimmermann, @jerome
/reviewed-on !43 + slapos!1863 (comment 260064)