Limit pytest version for LTS

[bsipocz]

Pytest 4.0 is not compatible with LTS as we can't really do much about it as we cannot remove API from this branch, so the workaround has to be to add an upper limit to the version dependency.

See: #7782 (comment)

[nicoddemus]

I agree this seems like the appropriate solution! 😁

[pllim]

LGTM (as long as the tests can be made to pass).

[bsipocz]

Re passing tests: this branch has a circleCI issue for a long time on 32bit that we don't understand (but it's somewhat transient, and doesn't seem to be astropy related).

But the rest of the failures is due to astropy/ci-helpers#337, so I think I would just pin pytest version for those to make them pass (pip install perfectly takes the version dependency into account, so for the users it shouldn't be such a big deal?)

[bsipocz]

All but the usual 32bit is passing, so I merge this now.

[olebole]

When Debian updates its pytest to 4 (currently, it is 3.10), I will have the choice to either disable testing for the 2.x legacy code, or to remove it completely (... and so almost the whole Python 2 astronomy ecosystem).

BTW, Is there a reason why it limits to <3.10 and not <4.0? The comments suggest that 3.10 should be supportable, right?

[bsipocz]

pytest 3.10 also shows the issue: https://travis-ci.org/astropy/astropy/jobs/459029569#L4185, I suppose I should have done it more consistently and specify it in setup.cfg, too.

And yes, maintaining the 2.0.x LTS branch is starting to be rather painful for us, too but we can't do much about it if we intend to keep it a no API changing one.
But it's still self consistent with its bundled libraries and dependencies listed in setup.py. If distributions chooses to unbundle/ignore those, I suppose they can stop supporting the LTS whenever it gets too many version clashes? Users can still choose not to update their distros, or switch to use conda/pip?

[nicoddemus]

And following @bsipocz, just want to say that us from pytest's side hate to break things on user's side like this, but sometimes this kind of breakage is necessary for us to move forward.

[bsipocz]

Re @olebole - It seems that the pytest collection is broken for >=3.7 & <4.0, so it seems we need to limit LTS to pytest <3.7.
It seems we can't really do much about it, so settled on listing it in known issues.

Details: #8177

[olebole]

@nicoddemus since a number of packages seem to fail in Python 2 with pytest 3.10 (my experience in Debian; I could provide a list if needed): would it be possible to still have 3.X version that fixes the problems? That would allow me to keep the Python 2 Astropy ecosystem in the next Debian release.
I am however also fine with removing it.

[nicoddemus]

@olebole I would love to help here, but we don't have the capability in pytest to have a 3.X version now... our versioning system does not accommodate that I'm afraid. ☹️

[olebole]

@nicoddemus OK, thanks for clarification. Our next release (Buster) will be out not before summer/fall 2019, so it is perfectly reasonable to drop Python 2 support for Astropy.

[bsipocz]

And we will stop supporting python2 ~Novemberish next year, so this all comes together nicely.