Reset namespace for testing config docs

Currently doctesting has been switched off for ScienceState related docs, but ideally anything that suppose to work for the users should be tested. To make this happen we need to reset the namespace while testing, I’m certain there is a clever way to do it.

Bottom line, those docs should be tested by the time we ship it to the users, if nothing else by brute force removing the modules prior in a testsetup directive, etc.

See discussion here: https://github.com/astropy/astropy/pull/8517#discussion_r289228267

[embray]

Assuming this still hasn’t been fixed, I have some ideas for this issue. I think we can take advantage of some more modern Python features (e.g. PEP-562) to clean up some of this mess.

[bsipocz]

@stargaser - I strongly feel that these should be sorted out before 4.0 as the current way I don’t see how well the behaviour is predictable in e.g. a pipeline, aka we should provide reproducible examples in the docs.