get_argspec accesses the user object 3 times.  The first, ob.__call__ was already wrapped in try-except.  The second, signature(ob or ob.__call) is wrapped by this issue.  It also adds a new test based on Dan's example. The third is (ob or ob.__call__).__doc__.  I did not wrap this because I could not create an example for which this fails.  There seems to be some special casing of this special attribute so that its default is None.

I opened #40180 for the isinstance bug and #40181 for further get_argspec changes, in particular, removing the positional-only '/' note.