Revert "PKG: Set max pin for Cython (pandas-dev#35396)"

This reverts commit d3343d9.

Author: tacaswell

pyproject.toml

@@ -4,7 +4,7 @@
 requires = [
     "setuptools",
     "wheel",
-    "Cython>=0.29.21,<3",  # Note: sync with setup.py
+    "Cython>=0.29.21",  # Note: sync with setup.py
     "numpy==1.16.5; python_version=='3.7' and platform_system!='AIX'",
     "numpy==1.17.3; python_version>='3.8' and platform_system!='AIX'",
     "numpy==1.16.5; python_version=='3.7' and platform_system=='AIX'",