Update Cython requirement - #5498

Could you please also add PYTEST_HEADER_MODULES['cython'] = 'cython' to astropy/conftest.py? Or maybe even to astropy/tests/pytest_plugins.py, but I think it would be actually better to clean up the latter a bit as all the affiliates inherit that dictionary.

@bsipocz - done

Not sure but there is also a pip-requirements and pip-requirements-dev file, would it make sense to update these as well?

Or use these instead of updating the narrative docs?

@MSeifert04 - done (we should still update the narrative docs)

It seems posix.types was added in Cython 0.21: cython/cython@4345655
If it is useful to keep compatibility with previous versions, it is easy to avoid using posix.types. Otherwise thanks for updating the requirements !

@saimn - if it is indeed easy to avoid using posix.types, that would be a preferable fix, just to avoid reports of this issue when people accidentally use versions that are too old. Could you do a PR for this?

👍 - looks like the remaining CircleCI fault is to be fixed in #5497 ?

@eteq - yes, that's correct!

Alright, this looks fine aside from the other CircleCI issue, so will go ahead and merge.

One follow-up question (which is only relevant for the milestone, not the code): do we consider it OK to change the Cython requirement as part of a bugfix release? That is, this is currently milestoned for 1.0.11, but should it instead be 1.3?

@eteq @astrofrog : see #5501 to avoid changing the Cython requirement for 1.0.11