BLD: catch missing cython instead of tempita NameError
- [x] closes #28836

I named this branch "bld-misc" in the optimistic hope I could close a bunch of BLD issues, but nope, just this one.
Looks good to me. I find the global `Cython` and `CYTHON_INSTALLED` variables a little strange, so this definitely helps

Anything in particular you are thinking we need to improve on with this? > I find the global Cython and CYTHON_INSTALLED variables a little strange

Yah I think you're right that they are redundant.  I'd be on board with having just one of them. > > I find the global Cython and CYTHON_INSTALLED variables a little strange
> 
> Yah I think you're right that they are redundant. I'd be on board with having just one of them.

yep agreed