[py] consequences of allowing snapshots based on repr

[nedtwigg]

Python has a very cool repr() method, which returns the source code required to create the object. Not everything implements it, but Python's core datastructures do.

For inline snapshots it's pretty easy - we call repr() on argument, and put that inside the to_be() call.

For disk snapshots, we would have to read the value from the snapshot, call eval() on that string, and then hope that the comparison logic is implemented correctly.

In both cases, it muddies the concept of equality a bit, but allowing repr is so convenient that it's worth it.