python3Packages.leather: fix build•••Broken in 2f4ea0c6dfb067551bff9f04c10d7c9e8eb70dcf.
Upstream changed from nose to pytest. Previously, the
setuptoolsCheckHook would run and execute the `./test.py` file, but that
is not part of upstream’s build process and doesn’t work. Including
pytestCheckHook means that does not run—just pytest.