Update test command in README
Update test command in README

Update this command to run 'pytest', this follows the setup used within
the CI pipelines.

The existing command expects a 'unittest' test suite:
https://setuptools.readthedocs.io/en/latest/setuptools.html#test-build-package-and-run-a-unittest-suite

maciejkula/lightfm,lyst/lightfm