Use system pytest

[aphearin]

Now that #788 has been merged so that astropy-helpers has been updated to 2.0, start using pytest instead of astropy bundled version.

[aphearin]

@bsipocz and/or @astrofrog - now that I have updated to astropy-helpers v2.0 with #788, I am getting new UserWarnings that trigger test failures (possibly because I am using enable_deprecations_as_exceptions, although this could be a red herring). Although I am not sure why the warnings show up now but not before, this is perfectly fine, but the problem I have now is that I need to catch these warnings so that my tests do not fail. Can either of you suggest how I can accomplish this?

Note to @bsipocz - this PR replaces your previous #760, simply because I found it easier to just run a new recursive search-and-replace as opposed to rebasing.

[bsipocz]

@aphearin - The warnings show up now as pytest 3.1 changed the warnings policy, and now included in its core the previous pytest-warnings package. You can turn off the warnings with adding
addopts = -p no:warnings to the [pytest] session in setup.cfg

[aphearin]

Gaw, sorry about that @bsipocz - I should have caught that one myself; obviously I was just thrown off by the sudden appearance of the harmless warnings (which I think I will keep after all, as some of them point to testing functions that could be improved).