pyx.build: Fix build after Cython 3 release

Unfortunately the best Cython release ever[1] breaks build for Pygolang[2], and
people on the net also report that there are other problems - both build
failures and popped up performance problems due to changed semantic[3,4,...].

Fix it all on Pygolang side by, for now, not delving into being Cython 3
beta-testers and relying on field-proven Cython 2.

/reported-by @levin.zimmermann

[1] https://groups.google.com/g/cython-users/c/o41CrcRkVvo
[2] nexedi/wendelin.core!16 (comment 188623)
[3] https://news.ycombinator.com/item?id=36778617
[4] https://github.com/cython/cython/issues/5540