docs: update cython and setuptools RTD URLs (#19512)

Fixes docs build failure discovered in #19510.

Updated setuptools URL too because previous CI runs say that

> intersphinx inventory has moved:
https://setuptools.readthedocs.io/en/latest/objects.inv ->
https://setuptools.pypa.io/en/latest/objects.inv

Author: sterliakov

docs/source/conf.py

@@ -278,9 +278,9 @@
 intersphinx_mapping = {
     "python": ("https://docs.python.org/3", None),
     "attrs": ("https://www.attrs.org/en/stable/", None),
-    "cython": ("https://docs.cython.org/en/latest", None),
+    "cython": ("https://cython.readthedocs.io/en/stable", None),
     "monkeytype": ("https://monkeytype.readthedocs.io/en/latest", None),
-    "setuptools": ("https://setuptools.readthedocs.io/en/latest", None),
+    "setuptools": ("https://setuptools.pypa.io/en/latest", None),
 }