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

pyproject.toml

 [build-system]
-requires = ["setuptools", "wheel", "setuptools_dso >= 2.8", "cython", "gevent"]
+requires = ["setuptools", "wheel", "setuptools_dso >= 2.8", "cython < 3", "gevent"]

setup.py

@@ -155,7 +155,7 @@ class develop(XInstallGPython, _develop):
 # requirements of packages under "golang." namespace
 R = {
     'cmd.pybench':      {'pytest'},
-    'pyx.build':        {'setuptools', 'wheel', 'cython', 'setuptools_dso >= 2.8'},
+    'pyx.build':        {'setuptools', 'wheel', 'cython < 3', 'setuptools_dso >= 2.8'},
     'x.perf.benchlib':  {'numpy'},
 }
 # TODO generate `a.b -> a`, e.g. x.perf = join(x.perf.*); x = join(x.*)