Stop catching TypeError in groupby methods
cc @WillAyd @jreback 

Note this doesn't touch cython_agg_blocks, I've run out of things to meddle with there.

Open to suggestions for the DecimalArray case.
> don't we need to have tests for the complex path ?

we have a couple of tests that get here with an ndarray[object] containing complex objects thanks @jbrockmendel 