MAINT: loosen Cython pin in environment.yml

0.29.30 is the minimum version, but there's no need to pin to that
exact version.

This is a follow up to numpygh-21530

[skip azurepipelines]

Author: rgommers

environment.yml

@@ -8,7 +8,7 @@ channels:
   - conda-forge
 dependencies:
   - python=3.9 #need to pin to avoid issues with builds
-  - cython=0.29.30
+  - cython>=0.29.30
   - compilers
   - openblas
   - nomkl