This PR does the following:
1. removes old python sort API test
2. creates a new python sort API test, obtained by translating the (entire) cpp unit test https://github.com/CVC4/CVC4/blob/master/test/unit/api/sort_black.cpp
3. adds support for bags and datatype selectors/testers domain/codomain in the python API.
If building with python bindings, check the pytest is installed, and adds a command to run pytest after the existing make check tests. If built without python bindings, it just uses a null command. Note: the current semantics are such that the pytest tests will not run if the ctest run fails (unless you pass the correct flag to make to continue --ignore-errors I believe). I can look into changing this semantics if that would be preferred.
