A common pattern when using kgb is to check if a spy was called only

once, and sometimes once with specific arguments. Callers had to do this

by checking the call count and then checking the arguments, or to use a

spy operation that would fail if executed more than once.
This introduce two new functions to perform these checks in a simpler

manner:

assertSpyCalledOnce()
assertSpyCalledOnceWith()

There are also pytest equivalent functions for both.

All unit tests pass on all environments.