Spy operations that take list of expected calls can now define an op

key in a call for a nested spy operation. This allows them to take

advantage of useful ops like SpyOpReturn or SpyOpRaise. It also

allows for more complex rulesets that nest SpyOpMatchInOrder within a

SpyOpMatchAny, which is useful when there's a lot of calls to a

utility function with different arguments.
This will be the major feature for 6.1.

Unit tests pass for all supported versions of Python.