docs: update the simplified make alltests instructions to include reference to installing coverage

[kevinmarsh]

The "Alternative unit tests with pytest" instructions in CONTRIBUTING.md no longer work after switching from nose -> pytest. You need to also install coverage and pytest-cov in order to run make alltests

ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]                                                                                           
pytest: error: unrecognized arguments: --cov=tqdm --cov-fail-under=80