Using system pytest rather than astropy bundled - #491

Thanks, @bsipocz. Many travis tests are failing with "Fetching package metadata ...An unexpected error has occurred, please consider sending the following traceback to the conda GitHub issue tracker at:...". Is this a conda version mismatch issue?

@larrybradley - I've never seen that before, will investigate later today.

It seems that restarting the build will solve the issue (it worked for one build, now restarted all of them).

Thanks. All checks have passed now.

Do you think we need to add pytest to the requirements and thus changelog or is it coming with python by default?

In the installation docs we state that photutils.test() is a way to run the tests for an installed version, so at the very least we also should mention there that pytest is required to run the tests. Since pytest is required only to run the tests, I'm not sure if we should formally make it a package requirement or not. What will astropy do? 😄

The last commit here should fix the unittest issue we see in #532. So we either go ahead and merge this (I think it shouldn't matter whether upstream astropy/astropy#5694 gets merged or not), or that commit can be cherry-picked into that PR.

astropy LTS is not compatible with pytest3 (astropy/astropy#4607 (comment)), so I've changed it to run with python3.5

@larrybradley - Should we go ahead with this? Having pytest3 would solve tons of failures in #532.

@bsipocz Yes, let's go ahead with this. Many thanks!

It may not work with LTS, that case we will need to limit astropy's version for isophot testing. I'll keep an eye on CI here and in the other PR, too.

This is now depends on astropy/ci-helpers#184

@bsipocz I fixed the biweight failures, so if you rebase this it should pass now.

@bsipocz Is this ready to merge?