Update codebase to use new theano.gof.graph stack_search, inputs, etc., generator functions

[Spaak]

The API for theano.gof.graph.stack_search was updated (improved) in aesara-devs/aesara#259. This function is used here (and nowhere else in the PyMC3 codebase, I checked):

https://github.com/pymc-devs/pymc3/blob/240c372cf2709e1d3212aa5873b648aa6a6855cc/pymc3/model_graph.py#L72

At first I thought a simple change of argument names would do the trick, but this yields empty results. So I'm hoping @brandonwillard has a pointer on how to rewrite the old-style API to the new one :)

[twiecki]

This is a generator now, so maybe just wrapping this into list will do the trick?

[brandonwillard]

Yeah, wrapping the results of the new generator-based functions with list should do the trick; however, depending on how the results are used (e.g. a one-time call to node in inputs([graph]), it can be a lot more efficient to use the generator directly.

[brandonwillard]

I've just created a rel-1.0.15 release that can be used for this PR.

[Spaak]

Ah yes of course, I got thrown off by the return value not being used in model_graph (instead the ancestors are built up as a side effect of the _expand function), but also in that case wrapping in list should work. Thanks!