Added plugin to filter by sub-packages

[astrofrog]

This is a small plugin that was inspired by discussions in astropy/astropy-APEs#52 - basically this adds the ability to do e.g.

pytest -P io.fits,constants

However, this only works for now if I remove:

doctest_plus = enabled

from setup.cfg, so I need to spend more time to understand how to get the plugins to work nicely.

Note that this also makes pytest-astropy into a real package rather than just a meta-package. I think this plugin is too astropy-specific to put in a standalone plugin.

[pllim]

Is this really necessary? pytest can already support collecting only tests from given directories, and our subpackages are organized in directories. Wouldn't something like this already be sufficient?

pytest astropy/io/fits/ astropy/io/misc/

[astrofrog]

@pllim - I got asked several times in astropy/astropy-APEs#52 for this functionality, so just thought I'd give it a shot - @bsipocz and others listed it as a critical feature of python setup.py test that they can't do without :)

Note that this isn't just equivalent to:

pytest astropy/io/fits/ astropy/io/misc/

it's equivalent to:

pytest astropy/io/fits/ astropy/io/misc/ docs/io/fits docs/io/misc

so it does save a bit of typing.

I never plan on using this though, I'm fine with just calling pytest with the explicit paths.

[pllim]

calling pytest with the explicit paths

Explicit is better than implicit, right? Maybe they just have to deal with a bit more typing and less maintenance for us.

[bsipocz]

I have strong feeling about it that we need this, otherwise many would end up not testing the docs. And it's convenient, too. And honestly we maintain less useful stuff all over the place.

[pllim]

There's a conflict.

[astrofrog]

I split this out into a separate plugin package: https://github.com/astropy/pytest-filter-subpackage - so I'll close this for now, and once that plugin is released we can add it as a dependency for pytest-astropy.