SpyAgency now has snake_case versions of all the assertion functions,

which is more compliant with most functions in Python, outside of the

unittest assertion functions.
This is largely to help kgb feel more natural in testing environments

not based on unittest (like pytest).
Each assert_* function is also available as standalone functions in

the kgb.asserts module, allowing use without needing to explicitly

create or mix in a SpyAgency.

Unit tests pass for all supported versions of Python.