astropy: rst files are no longer tested

The current version of the testing setup does no longer test any of the code in the doc/ folder, and thus the code in the documentation could be out-of-date and we did not know. I noticed this for an affiliated package and originally opened it as an issue for sphinx-doctestplus at https://github.com/astropy/pytest-doctestplus/issues/24 but at this time it’s not clear if it needs to be resolved in astropy itself, astropy-helpers, sphinx-doctestplux, travis-ci or upsteam in pytest. In any case, it seems that pytest 3.5.x works in full, pytest 3.6.x runs the tests, and I don’t know what would happen on pytest 3.7 because that has another bug causing infinite recursion in the test detection. Thus travis-ci currently freezes the pytest version to 3.6.

This probably effects all packages that use the astropy test infrastructure (I checked astropy, photutils and marxs), but I won’t open separate issues for each an every infrastructure package and each and every affiliated package. This issue and the link to https://github.com/astropy/pytest-doctestplus/issues/24 with more details will have to suffice.